Waterfall is suboptimal because it's optimistic and lacks feedback loops. Waterfall as codified was not the improved process Royce described, but the flawed process early in his paper.

It's optimistic because it carries an assumption that you can delineate your development into clear phases with a distinct start and finish. That doesn't work on large projects. You don't spend years designing your new system and then years building it and then years testing it. You commingle each of those, and once you do that, you're not doing Waterfall. You're doing something better, you're using your brain.

Iterative & Incremental, Spiral development, or most ideas out of the Agile movement are better. They incorporate feedback into the project and don't have strongly delineated phases. They don't make 10 year project commitments before they've even written a single requirement. Because these are methods that are realistic.

We didn't get to the moon by Waterfall. The Wright brothers didn't succeed in flight with Waterfall. Linux wasn't developed with Waterfall. Waterfall is a failure for large scale systems.