All those things are true. It still doesn’t sound like 1000+ engineers at 350k/yr.
What actually happens in a startup is you encounter these problems one at a time as they arise.
Startups have no users and no data to start with, and if they fuck up security, well, they just fail sooner than expected.
Once you get past a certain size, you have very different sorts of problems. Any idiot can vibe code a facebook lookalike, but the real one has to handle hundreds of millions of users and posts while being a target for state actors.
TLDR; yes you do need that many
Twitter wasn't built by 1000 engineers at 350k/yr.
It had to hire them later on. Because when there are users - you need support, take out fires etc.
And this exact thing will happen with any homebrewed SaaS.
You either run a business or play tech company making your own saas instead of focusing on your business.
Sure you can do both in very rare cases - if you are SpaceX or similar, otherwise you are shooting yourself in the foot.