I think everybody has their combination of moves, or "trademarks". Whether its Undertaker's snake eyes into the big boot or CM Punk's running knee to the bulldog but its when a guy seemingly only uses those moves to win and it becomes a recurring ending sequence it becomes problematic.
I agree. Every superstar has a setup for their finishing; sometimes they'll get there differently (Punk does this often, so much so it's usually a surprise when he signals for the GTS).
I also only heard of the 5 moves of doom once Cena started doing it. I think what makes it "5 moves of doom
" is regardless if it's 5, 6, 1, or 100. What makes it "dooming" is that it's the only way they finish a match. It's not necessarily the finish of the match (specifically the winner's finishing move), it's how it got there. Cena doesn't win a match unless he does the 5 moves of doom, or at least the FU; that's the problem. He usually puts on decent matches however, which is then confusing.
How can he be so productive in the ring for 10 minutes, then just rely only on the 5 moves of doom? He obviously has more in his repertoire, but the reason he chooses the 5 moves is the mystery in the OP. That's really the question we need to ask. Bret Hart didn't have to do 5 moves to beat an opponent, neither does Cena; why do they always do it?
I think it's just for familiarity. You tune into RAW to see a story line, some wrestling, but more importantly because you're comfortable with this made up world (where the good guys always finish a match in only 1 way). So, like jonoaries said, it's a trademark that makes the one wrestler stick out; it gives you something to look forward to when watching, something to expect and crave.