I could see Pokemon Legends Z-A being the Switch swan song, one last AAA release for the giant install base. It could also be launched along side a new ultra-budget version of the Switch (like the GBA Micro and Wii Mini).

Pokemon games have come out for previous gen systems. Crystal (GB) did got its international release after the GBA. Same thing for Emerald (GBA) not coming out in most markets until after the DS. Black and White 2 (DS) came out in 2012 while the 3DS came out in 2011. Ultra Sun and Moon (3DS) came out 6 months after the Switch.

If Nintendo does allow a major release on a previous generation system, it could be a sign that their current/coming soon (post-Switch) system has robust backwards compatibility.