I also think the max attrition cap is too low. You can change it in the static modifiers (under land_province), and there are few other changes that I think help, like adding max attrition to the winter modifiers (and scorched earth, but I think there are better ways of handling scorched earth).
Still, winning wars through attrition is something that can only benefit the player, since it's easy to game the AI into stupidly wasting troops on attrition (and you can't really blame paradox for this one, I don't think it's actually possible to write an AI good enough to manage something like attrition alongside all the other decisionmaking without running it on a super computer).
In my opinion it would be better to let the player use attrition to their advantage, but it should be something that requires either unusual circumstances or preparation on their part. If you build special buildings that raise the attrition cap in a key province, scorch it, and maybe take a policy (I refuse to accept "pick an idea group and dump points in it" as preparation), then I think you've done enough to earn the advantage, but it shouldn't be like before where you could scorch pretty much any province in the world and win a war off of that alone. I actually have an unreleased mod that does this, and it seems to work well (except for the small issue that raising local max attrition also does so for friendly units).