> Because if you ever go back a step, it is no longer the waterfall model
By this definition there has never been a waterfall project. They all need to correct, adapt, or redo something.
Look, I think I get your point but you seem too stuck in some formal definition riveted with assumptions to understand the other side. If you build a house you can correct some things after the fact. You forgot a power outlet or a light fixture? You need to move or resize a window? Probably fine and even decently low effort. You realize your foundation is too thin or too small? You forgot about one room? You put the bathroom on the wrong side of the house? Not so fine and definitely not low effort.
The state of software development is so bad because so many devs just jump head first into writing code and in the end they deliver a pile of barely held together shiny bits that technically works but is a pain to modify, extend, maintain, secure, extract the expected performance, etc. All because there are 100 brick layers and not one architect or engineer on the team.
I have used waterfall for decades to ship millions of dollars worth of software. If you don't want it to work it won't work.