Corporate ones sure don't. Even outgoing HTTP is passed through proxies on most bigco networks I've worked at.

Yes. But I don't think that corporate firewalls matter when it comes to user adaption. If you need to work with SCTP in your company, you'll get your firewall rule, if not you don't.

If they want to only allow proxied HTTP, that's their decision and developers should respect that, and not mask everything under HTTP. Their administrators made a substantial effort to forbid everything, why would honest person try to overcome their effort?

Home routers is another matter, home users don't make conscious decision about it, but in my experience UDP works just fine (because a lot of games use it) and it should be good enough for any protocol.

Because in many cases, those administrators don't even realize they're breaking the network. They just run a botched config that they inherited from their predecessors or something like that.

You might say that they're clueless and shouldn't be allowed near networking equipment (and you might even be right) but it's not going to change a thing. For the foreseeable future, working around broken environments is all we can do.

It's not. Talk to them. If you really need port XYZ open because your task requires that, you'll get the port open.