being an aviation enthusiast does not, in any way, make you a better controller!
This is like saying being interested in music does not, in anyway make you a better musician. Playing music is just hitting/plucking/blowing the right note at the right time and anyone (or a computer) could pick out a tune with enough practice, but would you expect a soulful, virtuoso performance? In the same way a trainee with no interest could be taught to say the right words at the right time and have a full career doing so, but having an understanding of the subbtleties of what they are doing and why take a great degree of interest and enthusiasm. Some non-aviation enthusiasts have sufficient professional pride to learn these things (which paradoxically makes them enthusiasts?), but many don't and unfortunately I think these people will never give a virtuoso ATC service and some seem to start hating the job.
Back to the OPs question: -
In the UK when I was a trainee about a third of my course did not get though each stage of training. A few were given a second chance (more timing of places available than skill of the trainee) so maybe half of the course continued employment to endorsement.
I would wholeheartedly agree that training success is often increased with staff shortages. Wrong, but true.