Why's it unfair? Just because a player is under 23 it doesn't mean he was bought with the future in mind. Each of those players you mention have been signed to go straight into the first 11 (the today), just like pretty much every other signing Mourinho has made. I can't recall many occasions Mourinho has signed a player that hasn't been ready from day 1 and developed him. I can however think of several times he's signed ageing players as quick fixes though.
Mourinho doesn't care that Matic is about to turn 29, his focus always has been on the present and Matic has at least 3 good years in him. In 3 years he probably won't be at Utd so won't need to worry about replacing him but if by some miracle he still is, he'll happily just spend another £40,50,60m on somebody else to come straight into the side.
Reasonably priced is subjective but yea. Mourinho won't care about his limitations as long as he can do what Mourinho wants him to do. It's no different with Fellaini.