That's not a point here. They came back as better players from loans but still not good enough. If Todibo now goes to let's say Bayer, plays 50-60 games there in next 18 months (which is not a given I know but it's a best case scenario) and then comes back and fail to stay longterm as at least a backup would you say that's because of a failed loan or because he's not good enough to play at a top level club?
If you claim they were not good enough then clearly it is a point here. Moreover, it ignores the other point - that a player may indeed be good enough but the coach does not know how to get the best out of them.
What your hypothetical scenario fails to consider is that provided Todibo does go out on 'loan' or sale with buyback, it presumes that no other changes will be made to the rotation or depth of central defenders during that time and therefore making it less likely that even a player doing well at Bayer or elsewhere will even want to return. As I say, this has never worked out for any player. We see this with cantera players as well which is why some are considering leaving earlier on their own terms.
The biggest issue with Valverde apart from naive tactics is failing to rotate or use squad depth. We have one of the smaller squad sizes among the biggest clubs and yet the minutes are consolidated among fewer players in the squad than ANY of those clubs.
Given Todibo's lack of experience before even coming here and the fact that CB is outfield positon where most coaches (not just EV) usually rotates the least it's very unlikely Todibo will play enough to develop. So why not look for those minutes in another club? Yes, loan could not work out but him staying here and not playing also won't help him.
Lack of experience is irrelevant if a 16 year old can find a spot in the rotation (Helps that we otherwise have no healthy wingers). Moreover, that lack of experience in a big CL match away from home surely demonstrated enough to warrant further consideration if not minutes when a big question mark will remain on Umtiti. There's a reason why the club were after De Ligt, the alternative is to stick with 3 central defenders where one is a question mark? There is absolutely no logic to that, particularly when any potential sale of Todibo would recoup at best 10-15 million. Is there a central defender on the planet Barcelona could reinforce with at that price point? I'll wait but I am sure you know the answer to that
Either we have depth or we roll the dice with injuries, either we build for the future or constantly buy players and hope they will adapt when a central defender without experience will never integrate quickly to Barcelona. We are desperate for depth in central defense and finally we have a potential star, not just depth, there and right after he's given a chance he's linked away? Sporting wise, it's a disastrous approach albeit consistent from Bartomeu