Most developers have never seen a successful project

Is this bad science?  Since this is a retrospective study, there is non-random assignment for the treatment and control groups.  That introduces selection bias.  Projects that are more likely to implement a (long-winded) “waterfall” life-cycle approach are probably larger scale projects to begin with.  Correlation is not causation.  So, maybe it’s not the lifecycle approach that is the problem, but the confounding/lurking variable of project scale that is the problem.  The study should control for the size of the project to make a valid conclusion about success rate of the development approach used.  ie:  Building a large insurance processing system will use a lifecycle approach, while building a fitness app will not.  Apples to oranges, since one is much easier to be implement than the other.