“As far as those who won’t daven there anymore, it’s not that they’ve moved up, they’ve moved out of yiddishkeit entirely, that why they no longer daven.”
Not necessarily. Let’s say someone belongs to a shul with an OO rabbi with a non-kosher hashkafa. But the shul have an identity as Orthodox, there’s a mechitza, they daven with an Artscroll siddur, etc. Then the person moves for whatever reason to a new area, which like most areas, doesn’t have an OO shul (OO shuls are few and far between).
What do they do? Most of the time they will join whatever mainstream MO shul is available. At least on the surface, it will be closest to what they’re used to. And it will help them become more frum.