Don't limit this to SPAs, include the Jamstack, which has all the same problems, and the false promise that if you can statically render a few pages or parts of pages and put them on a CDN, everything will be fast. It won't, because to load dynamic content, you still have to do a lot of work and talk to a (gasp) centralized API over the internet.
SPAs and Jamstack favor developer convenience over end user experience. Let's have fewer loading spinners, and more SSR-by-default for pages with dynamic content.
But there's a cost savings, right? JSON requests for just the data necessary vs. sending the whole HTML page each time.
Personally I'd prefer to develop a website the old-fashioned way, but I see that the bandwidth savings is a major point for SPAs and if you're running a business...
SSR by default, and then if you want, client side hydration and navigation so subsequent page loads happen in the client. However, the size difference between an HTML response and a JSON response is negligible, and HTML responses don't have to wait for Javascript to download, parse, execute, kick off off a request over the internet, get the data back, execute the result, and update the DOM. Browsers have literally decades of optimization to show HTML to users as fast as possible, and doing this in Javascript is fundamentally slower.
Am I the only one preferring good old-school, "boring" stacks that I can run entirely on my own machine if needed and understand the sequence of operations as opposed to relying on dozens of third-parties, services, APIs, etc just to do what a stupid PHP script on shared hosting could do 20 years ago? I don't consider the modern complexity as convenience.
SPAs and Jamstack favor developer convenience over end user experience. Let's have fewer loading spinners, and more SSR-by-default for pages with dynamic content.