So this is where 12factor comes in - unless the application is operating at lower than layer 7 (and with some of the newer offerings even layer 3) there’s not much technology centric stuff going on. Everything Slack is doing is happening at the HTTP / gRPC / Websocket level, and it’s hard to make the case for self managed hosting.

The idea of requests going to an application server on a TCP or HTTP connection with the application server being able to access a database or datastore is now common enough for all the PaaS providers to have abstractions for it, with the load balancing and database/data stores being managed. If customers aren’t happy with the auto scaling logic that can be overridden, but it seems like everything else is pretty rock solid.

Slack’s USP isn’t to reinvent load balancing, distributed configuration or database management, so if I was running it and had a clean slate a PaaS seems like a better bet.