Just remember that for every Sancho that leaves to get more playing time and succeed there are a bunch of players like Diaz that leave and don't.
I believe that had Sancho stayed at City he would have been great, it just would have taken longer, but maybe he'd have learnt more in the process. Similarly had Foden left he'd also have great success, probably faster, albeit he'd probably have learnt less in the process.
The point is, the reason why Sancho is successful atm and why Foden will succeed is because they're both incredible young players with what I assume to be incredible work ethics and incredible mentalities. Wherever they went or go, even if they were actively being held back for no purpose other than to stall their careers maliciously (which is the worst case scenario that would never realistically happen), they'd both become great players.
The cream rises to the top.
If Sancho had stayed at City, I'm not sure that his development would have been that much slower than at Dortmund. He made 12 appearences for Dortmund in 17/18. Inititally they were from the bench, his first start was in Jan 18. Sancho is clearly a great option as an impact sub, coming on late in games and using his speed, strength and trickery. If he'd stayed at City I think he would have featured quite often last year in that role. If he'd done as well as at Dortmund, isnt it possible that we wouldnt have signed Mahrez? Its very unlikely that he'd have become a regular starter for us this season but he could still have accumulated plenty of minutes. Not as many minutes as for Dortmund perhaps, but higher quality minutes.