Earlier quoted context omitted.
Client side validation is for UX. Server side validation is for security, correctness, etc. They are different features that require different code. Blending the two is asking for bugs and vulnerabilities and unnecessary toil. The real reason that SPAs arose is user analytics.
> The real reason that SPAs arose is user analytics. Can you go into that a bit? I don't really understand what you mean.
But if you want advanced tracking, like tracking what a user is focusing on at a particular instant, you need to wrap the whole document in a lot of JS.
SPA frameworks came out of AdTech companies like Meta, and I assure you it wasn't because they had limited engineering resources.