At a previous company we had Series C financing and enough people to take over two floors of an office building... and our entire SaaS offering for hundreds of white-label business customers ran fine on a couple of big load-balanced EC2 VMs and one big Postgres database with some read replicas.
Yep, HA clusters/load balancing has been around for a long time and will get you pretty far even in some pretty large environments. Hell my companies main LOB app is running off a single Microsoft SQL serve(I know, I know, we have a lot of tech debt we're working through atm and it's on the list to set up HA). The longest downtime we've had was when we had to take it down to migrate it to our new hardware cluster and that was because the data transfer took 20 hours over a 1 GB port.
