Yea, it's only Edge, but if you look at WebSockets you'll see that everything except Opera Mini supports them - so you can get (some of) the same functionality without polyfills at all.
But why willingly introduce extra complexity to the system?
Websockets are way less trivial than SSE. Polyfills for a less popular browser seem to be a simpler solution to the problem, compared to websockets stack everywhere. Even if I don't have to implement websockets on my own and would use some mature library (because bugs happen even in such libraries).
