This statement is way too overly broad. JSON is terrible for tabular data, since you have to include the field names in each row. HTML will be much better for data heavy sites in this case. Also, as others have mentioned, you still need to download and process the JS needed to display the data (libraries, likely styli ng, etc.).
> Better UX for slow networks: even in countries with 5G coverage there are still a lot of corners where your device will struggle to connect.
1. Offline and autoconnect are lovely ideas, but most js devs don't care enough/aren't permitted the time to write with this sort of resilency in mind. In practice it isn't really an advantage.
2. When a SPA does fail, it is often worse than a back end framework. The browser will tell you the request failed and a very general reason why. Whether it worked or not is abundantly clear. Most SPAs just sit there and silently die, while you stare at the screen waiting to see if it will actually do something.
3. Most of the mobile devices you're talking aren't that powerful, so loading them down with a bunch of JS to run, makes for a really bad for UX.