Next is not MacOS so I think it’s a real stretch to claim that means long term support! Perhaps some of this is normal but I think Apple have made little effort to avoid churn and it suits them to keep things changing.

Apple have had significant churn in dev tooling, languages, sdks, frameworks, chip architectures over the last couple of decades. Perhaps that is completely normal but it does mean developers are always struggling to keep up, and an app written for iOS 1.0 needs significant alterations if not a rewrite to work with later versions. Almost everything has been completely replaced, from language to frameworks to dev tooling like resources.