This is impressive and would work really well in a lot of sites, especially ones with static content.

However it misses two important reasons to use a single-page app:

1. SPAs allow for highly interactive interfaces with custom components, drag & drop, audio etc.

2. SPAs can cache UI state in the client. The alternative is to store it in a server side session specific to each client. This is possible, of course, but it will either be lost every time the server is restarted, or it will need to stored in the database. Sending UI state back and forth to the database is another increase in load on the database and network.

There is also the more general point about SPAs exchanging less data with the server so putting less stress on the network but that is touched upon in the post.