Honestly I've never seen a waterfall model implemented without iterations. And you can always update the top-spec at later date if needed its just more difficult.. Which it should be.
I have. It didn't reach the end.
Our industry is well on the path to ignore the obvious truths for 50 years...
As an improvement, sometimes the project management *forced* the customer to make some checkpoints along the way, but they were a far cry from real iterations.