iOS 15 had a difficult time updating from 14 on my 6s. That’s a sign right there Apple is unlikely to support it under 16. Sad thing is there will likely be millions of those out there that are still usable. I personally use mine as a backup when I’m charging my X. Apple should consider providing maintenance support for iOS 15 on devices that still run it.I’d say stay on 14. There’s nothing saying that they won’t have a “required update” to 15 after 16’s release. They likely won’t reach back to 14, so you should be good there.