In general I think it would make it harder to change your team around.. and that would keep people's involvement in the game down and that would be a bad thing
Actually, it will probably do exactly the opposite.
I would love to put this to the test... I agree with DonkeyMan (no relative!)
To grow and evolve at the fastest speed involves a gradual rotation of playing staff. You are in favour of preventing this from happening and in turn deterring the most adventurous and dare I say it successful managers that others aspire to build their rosters like.
If we all keep existing players forever then there must be at least 15% of managers capable of training effectively with a divergence of lets say 5%.
When these teams then play with the paper, scissors, stone scenario we are back to a very basic simulation with luck playing the major factor.
There are fewer ways now to improve your team than there were 12months ago or should I say the options that are there make it somewhat slower. Lets not take away or reduce anymore variables.