The real NFL doesn't work like that either. High draft pick rookie QBs
rarely sit for a full season anymore.
The reason everybody talks about the season Mahomes sat and developed in KC is because it's pretty rare, and has been for the last 15 years or so at least. Not only did they already have a veteran QB in Smith that was regularly winning the division for them, but unlike most veteran starters, Smith actually gave a squirt of pee about Mahomes' development. Most NFL starters don't put that kind of attention into training their replacement...look at similar situations from Farve/Rodgers on up, and most of the time, the starter couldn't care less whether the rookie gets any better. Why would they train themselves out of a job?
Past that, there's just too much pressure to put the new toy in the game. Other than Mahomes, name a high draft pick rookie QB in the last 15-20 years who
actually sat a full season. Usually, they end up starting by week 8 at the latest. Watson. Mayfield. Rosen. Jones (even though Gettleman swore they were following the "Kansas City plan" when they drafted him). Darnold. Denver is about to start Lock as soon as he's available to play again.
Hell, Nate Jackson had a great description in his book
Slow Getting Up of the media, fan, and front-office pressure in Denver to start Jay Cutler his rookie year...and even though Jake Plummer had taken them to the playoffs for years, and had a winning record for the season at the time he was benched, as soon as he lost a couple games in a row, Cutler was in, and Plummer never played in Denver again. This isn't a new phenomenon.
So, be careful invoking "how the real NFL works" just because you'd like to be able to do something. A lot of times, it really
isn't, and hasn't been for a very, very long time.
It's ok to want the game to work a certain way without needing a "realism" justification for it....I think we as a community forget that pretty often.