This doesn't seem kind to people with slow (or expensive) network connections. Note that currently we are on the verge of a point in history where almost every

This doesn't seem kind to people with slow (or expensive) network connections. Note that currently we are on the verge of a point in history where almost every person, even in rural communities in developing countries, has an internet enabled mobile device. In the SPA model yes, they will have to wait a long time to download the initial JS payload, but that can be cached. Thereafter, if UI updates don't need to talk to the server, they can be computed client-side and the UI can update instantaneously. But in the suggested model, a much larger class of client-side actions would trigger (blocking?) network requests.