Strange is local, sure, and I'm sure they'll rebuild the sanctums before Avengers 5 comes along, but there were no wizards when Loki invaded, nor were they any when Ultron attempted to destroy the world. I'm sure they wouldn't have let the world go to ruin, but I just can't see them getting involved unless there's an outworld threat.
Wasn't it Wong that said (in the Strange film) that the Avengers defend against the materialistic threats, whilst the wizards protect against the mystical? If there's no mystical threat, would they get involved? Why haven't they previously? I can further see both Scar-Jo and Renner retiring from the scene too; Scar-Jo (may) be killed off, and Renner will likely retire back to his family again; he'll have lost them once, thus becoming Ronin, I wouldn't think he'd want to be away from them again, knowing that pain - assuming he'll ever remember it.
The West Coast idea is ... an idea, but when you've got portals and supersonic flight, and when you've got SHIELD everywhere, I don't think the MCU would really benefit from having different Avengers at different places around the world. Rogers and co were working abroad at the beginning of AoU already.