Not sure I follow entirely, but the short answer I think is that demons eventually fall off in penalty is to make sure you don't get to create one single super demon that wins the entire game for you: eventually, it falls behind in levels and isn't super anymore.
Uniques are a good example of this: many uniques have great resistance packages and/or stats that you can, through Fuse Demons, make into real powerhouses. I don't mind you getting to use these for 10 or even 15 floors, but at some point they have to fall by the wayside or you end up with everyone beating the game with Headless, Ccoa, and Friar Rush as their main party.