I personally use Matrix for text and Mumble for voice, but most probably won’t go anywhere. Discord is convenient, and most users won’t care enough about some extra ads to put in the effort to migrate.

Having globally routable IPv6 addresses for each device doesn’t prevent you from running firewalls.

On the tech side of things, the environmental impact of running traditional, centralized services is inherently lower than running any cryptocurrency off of a blockchain. To overcome the technical limitations would be to create another centralized service.

But yeah, there are almost certainly ways that traditional currency can reduce their environmental impact, too.