There's a big difference, though.
Guilliman can't protect psykers from Chaos, so he can't rule without the Emperor. The Emperor is needed.
The ethereals, on the other hand, are worse than useless: they don't know how to deal with daemons, they're allowing psychic races to feed a chaos god, and punish those who try to fix the situation. Whereas Farsight knows about this stuff and also knows how to banish daemons. And he's willing to do something about it.
The ethereals might have been useful at some point but they've far outworn their welcome.
Also the ethereals allowed at least two genestealer cults to take root in their own empire.