To be clear - I don't argue that it's impossible to run Twitter with smaller crew. Hard to say without a look at the internals, but if I had to bet - yeah, probably there are inefficiences. But OP was coming from "you can leave it and it will just run by itself" angle, which is very much different thing.
On one hand I totally get your arguments, it's what I would expect my engineering team to say and they are often right.
On the other hand I am watching this space utterly intrigued if somehow he manages to pull it off.
If he does it might say more about the engineering team left than him though.