It will be really hard to break into United in the years to come - as it should be.
If you are a central defender (and we sign Varane or Torres), you basically have to be at the same level as Lindelöf and better than Tuanzebe to have a chance. No way that will happen without several highly successful loan-spells.
As an attacking player you are (probably) up against Sancho, Greenwood, Amad and Rashford + the striker we will sign next season to replace Cavani. That could easily be 3 out of 4-5 attacking players in the England squad in the years to come.
As a left back you are competing with Shaw, as a right-back you are competing with AWB. As an attacking midfielder you could be up against Bruno, Lingard and Pogba.
Central midfield at the moment looks easier, which is good news for Garner.
There are in my opinion 12-14 players with the talent to be good enough to get into the United first-team squad- but realistically, only 4-5 at best will make it. So Iqbal, Galbraith, Garner, Hansen Aarøen, McCann, Hugill, Fish, Mengi, Laird, Hannibal, Elanga, Shoretire etc etc - only 1/3 of these at best will make it at United.