Workforce market forces like that have a vastly greater effect than “bandwidth optimisation”.
My evidence for this is simple: every SPA app I’ve ever seen is two orders of magnitude slower than ordinary HTML would have been. There is almost never a bandwidth benefit in practice. Theoretically, sure, but every app I come across just dumps half the database down the wire and picks out a few dozen bytes in JS code. There's a comment right here in this discussion advocating for this! [1]
Another angle is this: if you had a top 100 site with massive bandwidth costs, then sure, converting your app to a SPA might make financial sense. But instead what I see is tiny projects start as a SPA from day one, and no chance that their bandwidth considerations — either cost or performance — will ever be a factor. Think internal apps accessed only over gigabit Ethernet.
I’ve seen static content presented as a SPA, which is just nuts to me.
[1] "It's much better ux when a user downloads the whole catalogue" from: https://news.ycombinator.com/item?id=44688834