If the total number of perks must remain restricted to 10, why not allow perk exchanges at level-ups after level 11 (i.e. relinquish an older perk and get to choose a new one to replace it)? That way, your interest in veteran fighters would be better sustained, and the process of building them up would be more organic (i.e. you could keep choosing whichever perk seems most helpful in the current circumstances, thus saving lots of worry about the long-term viability of your pick against unpredictable future challenges). This would also increase the interest/usefulness/viability of some of the more specialist/situational perks. Plus it seems realistic that veteran fighters would remain capable of learning new abilities relevant to the challenges currently faced, while being more liable to lose older ones that are rarely relevant anymore.
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