You're right! If we just implemented everything correctly, there wouldn't be problems!

Anecdotally, every team I've worked with has converted to a SPA, and the main ergonomic upgrade is that updates to the signup page or the "not found" widget quit getting forgotten whenever we change tweak our design. There always seems to be one page that's out-of-date for 2 months before anybody notices, then it quietly gets fixed up because we forgot that the design tweak was a CSS+HTML change, not pure CSS.

Not that that's perfect -- once you have a SPA, nobody remembers the 404 page, because it's a different HTML base...