> The long and short of it is; when you build a front-end to a website, and you know it's going to be served over HTTP2...

Well, that's the rub, isn't it?

When are we going to possibly be in a state where we know our website is going to be served over HTTP2?

Not for a while, probably? Not just wait until all the browsers support HTTP2, it's wait until the browsers that _don't_ support HTTP2 are a small minority.