A squad under development can have good improvement and starts to get better results but surely they won't be able to perform well and get good results too consistently. Inconsistency means it's very dangerous to bet on such squad's performances as their performances are quite unpredictable so are their match results.
An example in the last season is Chelsea that were under development and even they finished in top four, betting on Chelsea's matches last season is quite risky bet decision and this is even more risky to believe in Manchester United and bet on their matches. They are in a worse quality, performance and consistent situations than Chelsea.
I understand your point, but one thing you most understand is Manchester united performance seems quite unstable. However, other teams might have poor performance in the early stage but before you know it they would start picking up again, just like as you mentioned Chelsea because they were totally not in good condition at the early stage of last season, but just before the end they were able to achieved a top four position, which is quite impressive. But Manchester united was in poor performance from the beginning to the end, even though they have start again this new season and before you know it they might likely continue like this till the end of the season, which is very wrong at least they would have started the new season with more winning ways.