The idea that anyone, good or bad, is "entitled" to their slot is part of the problem. It should be up to the club organiser(s), who in most cases are risking their own money, to decide who gets to sing.

Snail, I could have pointed you towards a number of clubs which fit that description, but as I have stayed well away from them for several years because they were so rubbish I have no idea whether they are still going. If your experience of clubs is otherwise, count yourself lucky.