Other projects can't use waterfall development because they would like to actually produce something useful instead of what was decided at the start of the proj

Other projects can't use waterfall development because they would like to actually produce something useful instead of what was decided at the start of the project. It's a whole different world of software development. If you set out to build flight control software because it is needed to run on a new airplane, you're not going to pivot midstream and build something else instead.