Where do you propose database lookups get done in the JS+API world?

Surely they are getting done on the server that hosts the API? Unless you are using something like Firebase or SupaBase which can sort of enable client side database access (but not really, it’s still doing it on a server, just their server and running the call through RLS or similar)

You still need to do it, but now you additionally have a serialized data format in between client and server. Now you need to keep that format versioned and backwards compatible, otherwise users with cached clients will break.

We need to do that anyway, our customers call our application through our API. Building our web apps on top of the same ensures that they have the full functionality available, kind of eating our own dog food.

(but yes, we have unusual customers I guess)

Indeed, the longer you are around you see these cycles repeat again and again.

Thin client vs thick client has been a point of contention since mainframes and minicomputers.

When I read that I felt grateful to not work wherever this person works

You've still got to do DB lookups to serve the API, except now your DB traffic has to go through more and longer pipes?