Like the Betamax recorder you bought when it came out, those Marconi shares you thought would provide for your old age, and that Sinclair C5 that seemed the perfect low-cost run-around, sometimes you have to accept that investments don't work out.

Might we be in this territory now with Jos Buttler? He is 29 now, after all, and playing his 41st Test. You might expect him to be somewhere near his best. Instead his performances are tailing away. Since the start of the Ashes - that's 10 Tests ago - he is averaging 22.05. He has made one half-century in 18 innings and has not reached 30 in seven innings.

All players go through tough runs, of course. The art of selection is to keep the faith with them until they come out the other side. There have certainly been moments in Buttler's career - not least on debut in Southampton, or in Kandy at the end of 2018 - when he looked a very promising player.

But promises need to be kept and investments need to make a return. And the Buttler who was selected to be positive and carefree and aggressive is starting to look ever more careworn, uncertain and weary. He looks, like Moeen Ali before he took a break after the first Test in the Ashes, as if he's struggling to remember what he used to enjoy about the game. He looks, admittedly from something of a distance, as if he's fallen out of love with it a bit.

Whatever happens in his Test future, Buttler is a special player for England. He has scored five of the quickest 11 ODI centuries ever made for them - including the fastest two - as well as their second-fastest T20I half-century. He has already played a huge role in winning the 2019 World Cup and, if England are to challenge in the T20 World Cup later this year, he will surely have to contribute again.

But you wonder if the struggle is starting to wear him down. The disappointment, the tension, the travel - he has a young family now - can all rob the game of the freshness and wonder it once had. You wonder if the attempt to turn Buttler into a Test cricketer - the sprinter trying to run marathons - might have robbed him of just a little of his lustre.

Part of the problem with Buttler is that he doesn't have much of a track-record of scoring runs in first-class cricket. He averages just 32.35 in a first-class career that stretches back to 2009 and, in 106 first-class games, has scored a modest six first-class centuries. Chris Woakes and Adil Rashid, by comparison, have scored 10. They both average more, too. Jonny Bairstow (43.51 at first-class level) averages over 10 more. In many ways, expecting Buttler to move up a level and suddenly discover a way to score runs was naive and unreasonable.

Increasingly the white-ball and red-ball games are different animals. In England, at least, the white-ball games are played on pitches so flat that batsmen have little concern for the seaming or swinging ball, while the red-ball game demands a strong defence and a tight technique.

Buttler has never really developed those skills. Instead, there was talk of relaxing and trusting his undoubted natural talent. There was the memorable slogan written on top of his bat handle and an acceptance that he should come in at No. 7 - which is unusual for a specialist batsman - with a view to punishing tired bowlers and an older ball. Time that might have been spent learning to play the moving ball was spent - quite understandably - learning to master the T20 game in the IPL and elsewhere. England prioritisation of white-ball cricket meant sacrifices had to be made.

And it's competence that breeds confidence. For if you don't have a decent technique and gameplan in first-class cricket, you will eventually be found out. Batting at No. 7 is fine, but attacks take second new balls and bowlers, at this level, often have remarkable fitness levels. Put simply: it just hasn't worked out.

Some will argue that Buttler has been asked to fulfil roles that don't suit him at Test level. And it is true that, in the early stages of his recall in 2018, it looked as if he had cracked it. He averaged 52.5 in the first nine innings - seven of them as a specialist No. 7 - before he was promoted up the order a little to accommodate the inclusion of Ben Foakes - Jonny Bairstow was injured - and then Bairstow. He averages just 26.82 at No. 5, compared to 39.69 at No. 6 and 31.77 at No. 7. He was also asked to take back the gloves.

You could equally argue that he has had every advantage extended to him. When Ollie Pope, for example, came into the team, he was obliged to bat at No. 4 - a position he had never previously occupied - to accommodate Buttler, who was a specialist batsman batting, at times anyway, below the keeper, Jonny Bairstow and the main all-rounder, Ben Stokes.

Bairstow was moved to accommodate him, too. Ahead of Buttler's return, in May 2018, Bairstow had scored two centuries in his previous five Tests - one at No. 6 and one at No. 7 - before the return of Buttler destabilised him. He has been asked to bat at No. 3, No. 4 and No. 5 since and averaged just 23.66.

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In truth, all players have to be a little bit flexible for the good of the team. So while there is no doubting Buttler's willingness to attempt whatever has been required of him, there have to be doubts about his ability to perform enough roles to make him anything more than a luxury player at Test level. And while he was recalled as a counterattacking No. 7, in this game he has scored slower than any of his team-mates who have faced more than 10 balls. His struggles to bat with the tail were shown-up by Ollie Pope, who did so masterfully in Cape Town, and his dismissal here - charging down the pitch and slogging in the air - was unworthy of one of England's finest natural ball strikers. And if the aggressive strokes are gone and the defence isn't there, well, what's left?

There are other options. With a tour of Sri Lanka looming, thoughts will surely turn back to Foakes. He was player of the series the last time England toured there and is probably the best keeper available. That will be an important factor with spin expected to play a major part.

It is true that Foakes endured a relatively modest County Championship season with the bat (he averaged 26.14) for Surrey. And it is true that, by doing so, he failed to make the most eloquent case for a recall. But might disappointment have been a factor? He started the summer by winning the player-of-the-match award on his ODI debut - in Dublin - only to be dropped before he had the chance to play another game. He has also never played another full series after that Sri Lanka tour. In 36 fewer Tests than Buttler, he has the same number of Test centuries and a batting average of 41.50. He could be forgiven if he felt somewhat hard done by.

Bairstow will be considered, too. But many of the arguments levelled against Buttler could be levelled against him too, and it is not as if he has scored a mountain of runs since he was dropped. He is looking better in training, however, and England may point to the example of Dom Bess for the improvement a player can make without actually appearing in a game. It may also be relevant that Bairstow made a century from No. 3 in the Colombo Test little more than a year ago. With Joe Denly failing to secure the role, it is that position that Bairstow must be eying.

This is not the end of the road for Buttler as an international cricketer. Far from it. Freed from the demands of the longest format, he could concentrate on his limited-overs career. He is already one of the best - if not the best - white-ball cricketers England has ever had. With more chance to remain fresh, there is no reason he cannot sustain such form for several more years.

We are in another World Cup cycle now, too. If Eoin Morgan decides he is unlikely to last another four years - and he is 33 now, so it could be a stretch - it may make sense to appoint Buttler as ODI captain in the near future. That way, he would have time to grow into the role and a new England team would have time to evolve around him.

Jos Buttler can still be a great England cricketer. Just not a great England Test cricketer. That's not so bad, is it?