To be honest Tony Stark gives a rather half-arsed explanation as to why he can't fund the Avengers anymore and why he's leaving the Mansion a burned-out pit.
After the Avengers didn't believe Tony Stark was being mind controlled (they dismissed him as drunk) after he's been a benefactor to the team (funnelled half of his personal fortune into the Avengers enterprise), when Wanda went batshit, I'd have walked away too. Sometimes, you need to step back and recuperated, that's what he was doing here.
It's just one of the few times a character we are supposed to sympathize with, if not totally agree with, says that crazy people are just too weak to be sane.
I hear you. But Wanda has a way of just... freaking out (I remember reading Avengers in the 80s, 80s) and then bouncing back. Wanda is also (well, was also, I haven't read Mighty Avengers since she tapped various members and founded the new team), so Wanda does have her moments of sanity when she's proven herself to be a valued team member?