So there's a couple things you're wrong about, first of all, you can absolutely downgrade your OS on your current MBP, just because you updated to macOS 26 doesn't mean that's permanent, it's very easy to downgrade any Mac. And second, yes the new MBA will definitely have the option to upgrade the RAM, you are incorrect in thinking they "only come in 16GB" as they currently go up to 32GB. Not sure why you think 16GB is the max. Every MBA has had that option to increase RAM so there's zero chance they'll remove that with these new ones. Lastly, 16GB is very good and enough for the vast vast majority of users, it's unified memory so it's better and more efficient, and if there ever was a moment you needed more these Apple Silicon Macs do memory swap with the SSD, so I think you would be just fine.