> it’s very possible to run a web app backend within ~10ms RTT of most users

Only if your users are all located quite close to each other, or (sadly very common) you only care about making it fast for US users and screw everyone else.

(Of course you can have "intermediary backends" around the world on a CDN's edge network or similar, but at that point you're paying the same complexity cost as this style of putting the "intermediary backend" on the client)

10 years ago, I was headlining a project where we had strict performance requirements (ssr, php mind you) - the target for any operation except login was 30ms, and any endpoint taking more than 60ms in one of the devs server would have to have explicit approval. Add the nework RTT if you dont want to have a local backend, and for most geographies is still well under 300ms. Fun fact, we actually designed a system that could be easily replicated between regions and/or perform internal routing, leveraging the operator network. 3 AWS regions would make the RTT of a request well within <100ms on average for >80% of the world population. Requests were mostly "instant" - the big trick that did it (at the time) as to avoid reflow in the browser. Funny how the spotlight is now on doing stuff that "solves" that problem with the technology that was designed to solve that problem.