If clients supporting multiple protocols become popular they could default to a sensible open protocol. Which, as the default in the popular app(s), would become the most widely used protocol. People can switch at their leisure because the app supports both, but over time the users transition because it's the default.

Since the old and proprietary protocols are only sustained by the network effect, they lose users, and eventually support for them gets dropped.

It's a method of transitioning from a collection of proprietary systems with their own network effects to an open one, by temporarily supporting both. For obvious reasons the operators of the proprietary systems don't want to be subject to that competition, which is they same reason they shouldn't be allowed to shut it out.

I think in the end, whoever chooses to block these apps(Trillian, Pidgin, etc) too hard will be the ones left in the dust.

Evidently that isn't the case.

yet

They are using Matrix. You can use any Matrix client to interact with Beeper's services.

A client that can interop with multiple services isn't adding a new standard.

The end result will likely be a few apps that do the legwork to beat the various messaging platforms into some common denominator feature set and give you a single pane of glass.

Unsiloing messaging isn't some quick fix. It would be like saying "we have all these forums like HN, Reddit, Twitter, SpaceHey, Facebook they should all share a global identity system and be able to be used all by a single app with the same APIs." Like we can't even make a generic ActivityPub client.