SPA gives you all the state stored client side, backend can be reduced to simple stateless functions. Anything persisted longer goes in the DB, and plenty of solutions exist that automatically keep the client side view of the DB and the backend in sync automatically, even across temporary loss of connectivity.

Compared to the bad days of server side session management, SPAs are far simpler to code and to reason about.

The elimination of the back and forth between client and backend means far less testing, and far simpler testing.

It is also arguably more efficient. Yes it uses more resources on the client compared to doing everything server side, but cpu and memory needs to be used no matter what. Treating web browsers as dumb terminals when the cheapest smartphone is at least quad core is a bit silly.

That said, there isn't any excuse for bad client side code that murders CPUs, but not using the client for anything but JPEG decoding doesn't make much sense!