Well, I bet the warden will be to op, plus it doesn't drop anything of value so that if you do defeat it, you get nothing.

The wither and ender dragon could just fly up and shoot them with whatever they have.

the wither would kick his ass

Probably not, even an iron golem could kill a wither as long as it has some positive status effects and both are enclosed underground, and if it doesn't, it would deal some decent damage to the wither, now take the warden for example which does MORE damage then a golem

as long as the wither is out of the warden's range he's fine

It's coming in 1.18