Waterfall came out when hardware and software had to be developed together, and appealed to traditional Engineering practitioners. You are right though, when the hardware constraints went away, software (more code) was cheaper and easier to ship in increments and iterate. But feature-rich products were still difficult to ship - and you had to pick and choose what things to spend your time on.

The SaaS-pocalypse is occurring versus investors don’t believe that to be true anymore.

I think they will still be wrong because ultimately people want people (particularly experts) to be held accountable for things - shipping high stakes software, running company ERPs/CRMs, and more.