What a surprise, the Rocket.Chat team leaving an open comms loop, even when there is potential to do a deal!
As for the scalability, in a former life I helped implement a RC cluster for chat functionality in a web application. Way less than 500K users, and a maintenance nightmare.
It’s unclear how much of this is to do with Meteor sucking as a platform, rather than RC sucking. They have been talking about moving away from Meteor for years, Moving away from Meteor and beyond - #8 by rodrigo.nascimento being an example, which, you guessed it, is left hanging in the air. Who knows if it will ever happen. I know where my money is.