Just maintaining and keeping an already built platform running at its current state doesn't require many engineers. So if that is your only goal then cutting your workforce to a skeleton crew will probably work for quite a while. It's only when you want to add new features, explore new markets and/or find new ways to increase revenue that all those extra people become.

I don't know what all those people at Twitter where doing, but I have worked at large companies and many engineers where working on developing possible new products and revenue streams, many of which, for different reasons, never see the light of day. If you're just looking at it from the outside it looks like there are a lot of people literally producing nothing.

With all that being said, I'm sure there are lots of companies out there (including pre Musk Twitter) who could lose a bunch of people without it affecting their bottom line or long term productivity.