We do similar but sprints are somewhat flexible, more like versions. We chuck the features we want from the top of most needed, split into stories and estimate like you mentioned using brainstorming between devs and QA. Estimation happens by relatively comparing complexity of each new story compared to previously implemented stories, conservativy picking average one up if there is variance in estimates. QA is involved to determine how long it will take to test the feature or sometimes what uncertainty is there if this is even possible automatically.
In the end we have stable developer velocity metric and a really close estimate for each new version.
As much as I agree with app deployment headaches, apps provide something a website cannot (except PWA) - ability to do stuff offline, log and register data which can be uploaded when connection is re-established. When talking about user experience - launching the app, selecting new -> quote -> entering details -> save -> locking the phone without worrying or waiting, knowing that it will eventually get uploaded, is much more convenient than walking with the phone around the property to get better reception to even load the new quote page.
UX matters, and user does not care if the native wrapper or 500kB of js is there or not, as long as the job is done conveniently and fast.
Am I missing something here? The mobile SPA app can be deployed using tools like capacitor to a device and the framework along with all static content is loaded into the app bundle. In such case it makes no (realistic) difference which framework is selected and it matters more how the background/slow transfers are handled with data-only API requests, possibly with hosted images. With the background workers PWA can be built as well, streamlining installation even more.
About that free market - it might be free but lobbying culture in the US starts early, at school where Apple pushes products to influence future choices of students. Not choosing something which is used by everyone around is probably more difficult psychologically than standing up to the status quo and choosing alternatives whatever they are.
There are choices made for use - having a phone on which everyone around communicates, having an internet connection from the only provider in the area.
Uniformity is a choice if the alternative is a viable choice and this is a complex matter.