Also worth noting that the entire stack isn't allocated at once so 1 million threads would be using 2TB/68GB of virtual address space, not 2TB/68GB of physical memory.

That is also the maximum stack size which shouldn't normally be reached. You'll have to be careful how you use memory when you're handling a million clients, either as async or threaded.

What "normal-sized Linux server" has 70GB of RAM?

How much does a 68GB cost on the cloud per day? Also, you don't have 1 million cores so quite a bit of your daily server costs will be eaten up by the OS running context switching code.

The difference, at least in the way this is built in Rust, is that when you create a task, you get a single allocation that's exactly sized. There's no resizing, which means that you aren't getting stacks that are too big or too small, with all of the other runtime shenanigans that that entails.

In some languges, that's true. In Rust, however, that pseudo-stack is typically tiny.