Legends is a mainline title because the developers literally tell you it is. There is no higher authority to appeal to, and anything beyond that is just going back to the old "well Super Mario 3D World isn't a real 3-D Mario", as if that means fucking anything at all.
As for Z-A, I'm intrigued since I really liked many of the changes and additions Legends brought, but also have some trepidation because I have a hard time wrestling with the shift to an intra-urban setting and how that seems like it would necessarily remove a lot of those same elements I enjoyed about the first Legends. The notion of needing to literally, physically avoid attacks from strong wild pokemon would practically need to be gone, as would a lot of the stealth elements and material collection and the whole vibe of venturing out into wild territory.
I'm trying not to develop any expectations about how this will actually work, but my hope is that the lessons they took out of LA weren't "people liked the idea of building up a central city, and also want way more trainer battles" and made that the focus over all the real-time world and pokemon interactions.