Even when IM started to break because it was too awesome not to try to monopolize, Trillian (back in 2000!) showed how interoperability was superconvenient to rein in the nascent feudalism and give power back to the users... until the bigger players put a stop to that.
The problem is that the legal system is misused to prevent adversarial interoperability.
Adversarial interoperability was never explicitly welcomed by gatekeepers, but back then nobody would think of suing over it, nor prosecuting such a case.
Nowadays, copyright law is used as a reason to shut those things down, even when the copyrighted content is actually third-party user-generated content or content you do have a license to access since it's freely accessible using the official client.
https://en.wikipedia.org/wiki/Trillian_(software)