So if you guarantee dodges on major tank busters, what would be the reason for taking any other tank

You must not have read my other comments. The trade off for being able to semi-regularly completely negate an attack would be that, outside of guaranteeing a dodge, the class would be pretty fragile. They could tank swap in to handle a tank buster or otherwise mechanics-heavy sections with no problem, but would need to be tank swapped back out because auto attacks would wear them down quickly unless they got lucky and dodged several of them consecutively.

A Dancer evasion tank in particular could also be particularly good at handling AoE mechanics since they could conceivably have several movement skills like Displacement, Plunge, and Shukuchi.

And because negating massive, run-ending mechanics is kind of OP, they could make certain mechanics undodgeable so evasion tanks wouldn't become completely ubiquitous.

I don't think any game that's done evasion tanks for MMOs actually enjoyed having them.

That's probably a fair statement, but that doesn't mean they can't try anyway. I don't think anyone that's put 3d movement combat a la underwater or mid air combat in MMOs enjoyed it, but that sure hasn't stopped people from continuing to do it.