I prefer my team grows healthy and gradually instead of dramatically. My teams comprise of all or large portion of young players. Their average qualities gradually improve during the season (increase by about 0.1Q per day except those days with squad changes). At the beginning of each season, relative average qualities are more or less the same, around high 4 star or low 5 star (actually slightly increase in each season) after promoting to next level.
With this cycle, I play with teams with similar qualities in each season, and can always enjoy competing top 4 or champion in league in every season.
First, for teams which do not promote in some seasons (I don't have experience), young players should be able to kept longer.
Second, good players can still be useful and kept even if its relative quality is dropped.
Third, players' growth really slow down at 22. But some of them are not too bad after 22. I have not carefully studied this but as I remember, some players still enjoyed quite healthy growth at age around 25.
I agree that the injury period in T11 is longer than real world. But there are red packs in T11 world. So it cannot directly compare.
I do not rule out all conspiracies about injuries. It seems there are more injuries when I use more red packs. So now I apply them very carefully and often let some players to sit out for a few matches even if I have red pack stock. My red pack stock is used to dropped to zero. In last few seasons, it is often maintained at 100-250 even though I don't farm any red pack.
In my experience, my teams suffered severe injury problems in some seasons but also had far less injuries in other season. You may be just experiencing a season with bad luck.
I do not aim at convincing you to stay. But honestly speaking, there are too many real challenges at higher levels you have yet not experienced if you just played a few seasons.
Anyway, hope you still enjoy the game in last few days of this season and may be try out something interesting here.