It hasn't been tested whether there's a cap on the CE loss reduction via enmity+ gear. So we don't know how high that can go. Heck, we don't even know how potent the effect is. Whether it's -1% per +1 enmity, or if there's some formula it goes through.

It doesn't really matter if you can't lose CE though. Once at enmity cap, the thing that determines who has the mobs attention, is who was the last to act. Naturally, there are more DDs than tanks. So the frequency of action from non-tanks will be vastly higher than that of the tank. So the majority of the time, the mob will be aimed at someone aside from the tank.

This holds true whether you ever lose CE or not. DD's will constantly recap CE even if they do take damage. Probably just via melee hits. If not, next WS would recap.

So while Crusade helps you hit cap faster, and reduces enmity loss, it doesn't address the issue that currently invalidates tanking.