Given that IE 10+ supports web sockets I don't really see the need for server sent events. What can you do with server sent events that you cannot do with web sockets? I guess one could deprecate SSE in favor of WS.

It's not what you can do, but what you have to do. Websockets are to Server-Sent Events as jet planes are to wheelbarrows. Yes, the stuff jet planes can carry is a superset of the stuff wheelbarrows can carry, but the cost is greater and it ends up being a suboptimal in many cases where you'd want to use a wheelbarrow.

Also, most people who think they want the semantics of websockets, really want the semantics of SSE. I've seen many, many webapps that basically build SSE on top of websockets.

SSE is trivial to support via a polyfill, and has none of the connectivity issues websockets has, so it's capable across a much wider array of devices/software both client side and server/network side

I feel that websockets is like the xml of this decade - "everyone is using it" so everyone wants to use it.

How can you polyfill it? Long polling JSON?

There are several polyfills linked from modernizer and I haven't looked at them all but I would imagine so yes. Because it tracks the last-read message ID, a broken connection is not a deal breaker