> I'm not convinced there's any real velocity gains in agile when factoring in all the fiddling, rewrites, and refactoring.

That’s not the point. The point is to end up with something actually useful in the end. If the artifact I deliver does not meet requirements, it does not really matter how fast I deliver it.

The reason waterfall methodology falls flat so often is not long delivery times, but ending up with completely the wrong thing.