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.