You can't divide the data on multiple servers (sharding)
It will come a time, when you'll exhaust the biggest machine they have available, and then you're stuck
While it's true that most companies will be fine in 256GB ram, we were talking about sharding, which it doesn't have.
I'm assuming [meme] is shorthand “overly-broad assertion”? It's nice if your database has horizontal scaling built in but it's not like we don't have an entire generation of successful companies who had application-level sharding logic either by necessity or because they found the control it offered was valuable compared to the built-in generic logic.
> While it's true that most companies will be fine in 256GB ram, we were talking about sharding, which it doesn't have.
You still haven't supported the assertion that it's common for places to have massive, heavily-queried databases like this which would not be better split following natural application-level boundaries. This is particularly relevant when discussing AWS as some of the common reasons for keeping everything in one database are sweet-spots for other services (e.g. migrating data for long-term historical reporting over to RedShift).
Again, I'm not questioning that integrated sharding would have its uses – only your sweeping assertion that this is a likely problem for most people and that it's a dead-end (“you're stuck”) rather than merely one of many growing pains which you'll deal with on a successful product. In particular, it's unlikely that everyone will have the same right answer at that scale since access patterns vary widely.