Companies might do that. They have the right to do so. If they still want to use that service, they will find a way to use it. Be it by vendor-coercing or simpler methods.

Just because those companies exist, does not mean that their shitty practices have any imapct on real internet connections. If you as a paying ISP customer want to use a custom port or whatever, it is going to work. So you as a developer don't have any restriction (which you don't know anyway beforehand) if you are developing a solution for a problem.

"Middleboxes" is a hackernews meme that is thrown around because people here work at places who restrict stuff and they can't bother to change that situation but instead complain about it.

The fact that games exist and they use all kind of ports is proof that this is not a problem for normal networks.