If a corporation comes to control the majority of traffic over a protocol through a proprietary client they can switch that client off of being able to communicate over the shared protocol and starve out the network - especially if they do it in a predatory manner[1]. It's not necessarily that Meta will try and acquire all the open source clients - it's that it will use the network to grow a starting pool of users and then switch protocols as soon as they can convert the majority of users to a proprietary client.

The protocol can be exceptionally well designed - and corporations can still effectively break it.

1. One example would be still publishing posts over the shared protocol but blocking the visibility of replies from people using an open source client.