Originally Posted by
Seerow
The main argument in favor of a big capstone is that it discourages multiclassing. No matter what you're doing, there's probably a prestige class out there that does what you're trying to do with the base class, that has some really cool unique features you can pick up by going into it.
By introducing a lategame capstone, you introduce a real tradeoff for multiclassing. Also, as you mention, so few games continue past 20, having a really cool gamebreaking capstone is largely irrelevant, so you can make that capstone exceedingly good and cool, without worrying too much about balance (and even if you are worried about balance, compare it to some of the stuff you get from multiclassing out and you figure 'eh, this really isn't that over the top afterall).