There's a big issue with letting players go over 20. With attributes 1 to 20 the progression is linear meaning the jump from 10 to 11 the same amount of skill added as a jump from 11 to 12 and 19 to 20. The amount of skill gained is increased further with each jump into the 20s. I don't know the exact calculations used but 20 to 21 is a greater jump in skill than 19 to 20. 21 to 22 is a greater jump in skill than 20 to 21 and it continues in that manner all the way to 40.

Someone with 25 in a attribute might have a 100% increase in skill compared to someone who has a 20 but the difference between 20 and 15 is only a 33% increase in skill. This would make parity between the best and worst players even greater.