It isn't a lazy assumption. Some managers like to give youth team players a chance and some dont. Mourinho is in the don't camp based on years and years of evidence. Signing the best young CB in the world and playing him sometimes isn't proof that he's going to continue united's youth policy. Exceptions to every rule. But saying something based on years and years of evidence is any kind of assumption is mad. A big mandate of his going back to chelsea was to make something out of their remarkable youth team, and he failed in the extreme.
Mourinho brings lots of good things, but to try to include youth policy in there is going to meet a lot of resistance. Reality is we are getting the best managers in the world at winning things - and sacrificing youth and attacking football to do so. If you think otherwise you're likely going to be disappointed in coming months