This is a difficult one. I know several pilots, both f/w and rotary, who were told early on that they wouldn't make it. Two of those people are now instructors themselves! People learn at incredibly different rates. It's very hard to be sure that someone will never learn. And 20-30 hours certainly isn't enough - maybe....60-70, don't know the exact figure, but some people are slow but will get there in the end.
The lack of airmanship is more worrying; in fact, that IS worrying, far more than the lack of aptitude.
Possibly there are some people who'll never make it, but there are far, far more who are told they won't by second-rate instructors.
That's all my thoughts so far after a very long day.