I think that a good solution would be to add some perks (extra) available only for veterans in your company. mercenary is, an example, in a party at least N days and no less than N level – he gets an opportunity to select an additional special perk.
So it will be possible to solve both problems :
1. “11 levels – not enough”
2. The trend to replace all the brothers for sellswords or knights in the late game.