A solution for this could be instead of +10 to skill use +2% of base AI to skill. A 500 AI player would get his +10 but U20 guys with AI ~250 would only get +5.
I just think it is not fear or if we compare to real life that you have boosted yourself at age 22 as new gun getting + attributes to energy, form, shooting or whatever and then when you are already falling star you still have all those atributes. In real life there are very few players that could keep up the skills till the end of career ( Jagr, Gretzky or Datsyuk ), but anyway they changed - speed changed to experience and decisions, decisions affected energy etc... Of course I have no problem if everything stays as it is, but in long term it does not make sense and is not real :)
Seasonal jerseys would be better. They did something in that season to earn it, not this season. Do again this season if you want a shirt, not have some pity stinky shirt from years back :)
Shirts are very difficult to win. 2 players per week for the entire league win it. The average team will have 5 or 6 winners in and entire year in real life. The boosts are already very small, seasonal effectiveness is far too small a boost in my opinion and I won't really be excited if my player wins.
A % skill boost benefits the teams with the best players and makes it harder for lower teams to catch up, since their lower AI players will gain less.