For larger volumes of events, we wouldn't recommend using Postgres.
The nice thing about single-tenancy is that in reality lots of users have small enough datasets that scaling isn't a problem. Heap et al have to scale to all of their users combined (as you said, terabytes), we just have to scale to the biggest user. Postgres also allows you to get started very quickly and do lots of queries yourself.
In our docs we explain our thinking more. Postgres is great for the vast majority of use-cases, and we're working hard to optimise those queries. Once users get beyond Postgres, we have integrations with databases that can scale well across many hosts, and we provide services around this to help people size their servers correctly.