Just the idea of a smartphone OS being around for over twenty years.

I think we're on the cusp of a lot of rules changing in that regard. The stagnation of Moore's law would result in some degree of stagnation in software architecture.

We're at the mercy of chip process engineers to a greater degree than is generally recognized.

Well I would have said the same before the last versions of Android have aimed at paving the way for the 10 years to come.

Stuff like project Treble have taken most of the engineering time on that release .. it seems that at least for now the plan is to still have Android in 10 years

(it should be noted that Fuchsia and Flutter are AFAIK teams that are completely separate from the Android framework team)

And well, the state of the OS is .. ok I guess ? Like all projects of this size, building APIs is building future regret.

Some can be rewritten. Some are so bad they are revamped. Some are meh but not bad enough to warrant the trouble of changing them.

The smartphone market has stabilized. And once the hardware stabilizes, the OS almost never changes. Source? OS/360, MS DOS, Windows.