No. Anyone can pick any communication network, as long as it is willing to cooperate.
In an ideal world where everyone's allowed to communicate in whichever manner they'd like, one should not only be free to choose any client and any service, but also any communication network. And in this article, I will explain why this is not the case.
This article is an expansion on the decentralization topic from my article on decentralization as a legal concept, on which I have also made a YouTube video. You can view the video down below. In this article specifically, I focus more on why forcing companies to join an open communication network isn't moving the goalpost.
If a network is open to all services to join, then it's also open for people from different networks to join the network's communication. Let's say that a friend of mine and I are a big fan of the Matrix protocol, and that two friends of ours prefer to talk on the XMPP protocol.
See the example in the image below. My friend uses the FluffyChat client, I use the Element client, and we talk to each other on Matrix servers in the bottom right. Our friends use XMPP clients (that I don't know) and communicate on XMPP, as depicted in the top left.
In this case, the network effect where either protocol is as useful as its popularity, still seems to be the case. Yet, the concept that both networks are open networks, provides us with a solution.
Since both networks are open, anyone can write software that communicates with either network. Even better, anyone can write software that communicates with both networks, and that functions as a translator between the two protocols. These "translators" are also called gateways, bridges, portals, whatever. Regardless of how you would call them, anyone can build a server that connects the two networks to each other.
If we connect this hypothetical server to both the XMPP network and the Matrix network, then everyone from both networks is capable of contacting other people in either network. Check the image below, everyone's connected to each other - sure, some people have a few more tomato cans between them than other people do, but that doesn't matter much.
Once you realize that you can connect all open networks with each other, you find out what kind of endless possibilities there are. A network like XMPP already has these "translators" with ICQ, AIM, E-mail, SMS, IRC, Yahoo Messenger, you name it. It's already possible to log in somewhere and then receive an SMS from that website. And why? Because SMS is an open network, anyone can send a message to you from anywhere as soon as they've got your phone number.
And we can even push this translating from one network to another even further: you can create your own network, and then build a translator from your network to any other open communication network, and you can then communicate from there with everyone else. This could lead to crazy schemes like the one on the picture below, where both you and your partner share messages that goes through a wide variety of open networks, but that's fine. You nor your friend would notice, and computers don't care.
No - moving to an open communication network is not moving the goalpost. Not only does the user gain the benefit that they can choose any client and any server, they can pick any communication network, as long it can connect with an open communication network that eventually can communicate with their friend's open communication network.