> introducing new clients for feature subsets (e.g. something running on a Garmin watch) becomes trivial.

This never happens, for some values of never.

When a SPA app is initially developed, the "client" and the "API" are moulded to each other, like a bespoke piece of couture tailored to an individual's figure. Hand-in-glove. A puddle in a depression.

There is absolutely no way that some other category of platform can smoothly utilise such a specialised, single-purpose API without significant changes.

The reality is that most SPA apps are monoliths, even if the client app and the API app are in different Git repos in order to pretend that this is not the case.

That's absolutely not my experience from any of the SPAs I've worked on. If the API used by the SPA is so "specialised, single-purpose", then that isn't something inherent to SPAs, only inherent to developers (or architects) who are inexperienced or bad at their job.