I once worked at a place where (for involved reasons not worth rehashing for this conversation) some of the backoffice, admin pages got rewritten in SPA fashion, after they had already been written in server-side fashion. It was an accidental experiment that showed me that the backend code did not decrease in size, whereas the frontend ballooned up to the same size as the backend (for fairly simple functionality even).

Now, this means you will need approximately twice as many developers. On the plus side, more work gets done on the client-side, so you probably need fewer servers for your billion users, and you may save in servers (and sysadmins) more than it costs you in developers to handle approximately twice as much code.

Except...99.9% of the shops using SPAs don't (and never will) have enough traffic for that tradeoff to make sense. The devs want (or at least wanted, back in the day) to go work at a company that did, so they wanted to use tools that would make sense if you're Facebook-sized. But, for the great majority of shops that made SPAs, there was no good reason to do so. This isn't Facebook's fault, but it is a reason why SPA's are annoying to so many people; they are almost always used in places that don't need them, and shouldn't be using them, in order to pad out a resume.

Cost is one factor, but what about user experience? Rich front-ends that users spend a lot of time in can save them time and the company money. Case in point, I work at a company where one team works on the front-end for customer support employees. They have to run through certain legal processes from time to time, like sending customers emails with contracts and the like, or looking up various data from various services. Their efforts have helped save these agents a lot of time, and part of that is putting more and more into a rich, clear and convenient front-end.

Of course, a lot of the effort also went to tying together various systems, replacing outdated ones, developing smarter and better chatbots and voice bots to guide users towards answers or self-service, etc.

I completely agree that SPAs are overused, but I also disagree that they necessarily balloon in size. An SPA is supposed to be a view, it is only supposed to concern itself with view logic. People just overcomplicate them endlessly with unnecessary crap.