> almost nobody else uses or understands enough to build on

Tools like Akka are very much alive and well.

> t feels like it's likely that if they merge with Microsoft they'll get rebuilt internally

I doubt that, unless they're using a lot of proprietary GCP tech. A Microsoft-owned business using Azure to (most likely) save money is kind of an obvious move, and I don't consider it a bad thing.

> their infrastructure replaced with Teams' infrastructure.

Or maybe even the other way around? Discord's tech is no slouch.