Yes, we have to evolve as the game is changing.
For example Academy players, recommended players, scout players aren't reliable to build a team because as I said in another thread, they are

Not in the right time
not in the right position
not in the right quality
not in the right price .
What is left are young fast trainers and negotiations.
Nego does not exist anymore and has as consequence to raise a lot the price (and bidding wars) for auction players,

I usually was keeping the players till 21 y.o. but as you can see the training also changed from old times, when a player above 21 was loosing a lot in gaining.
So the new strategy is buy young fast trainers, add positions and special ability, build their white skills high the first 2-3 seasons, then mix training (the whites anyway will increasing from the games and the grey decreasing from level up) and after 21 y.o., train the greys to pump up the general quality.
Mutants has no significant effect anyway after some internal programming the last period but quality and general quality still matters.
So keeping a player for 6-7 (at 23, stop invest greens for extra training) seasons decreasing the cost of greens to add positions and sa and you are saving tokens from auctions.