This post is quoting what Sup-ed Mad-io said about Bowser being a loser.
Bowser DOES pose a threat. Have you tried fighting him in Paper Marii: The Thousand-Year Door?
He doesn't always kidnap Peach, either. Or, whenever he does, he manages to do something else.
For example, take a look at Paper Mario for the N64; yes, he kidnapped Peach, but he also stole the Star Rod and planned to use it to grant evil wishes against the star kids. That idea isn't stale, is it? As far as I'm concerned, that plot has never been used before in a Mario game.
Paper Mario 2's story does include Bowser. but, he does not kidnap Peach; he just tries to take the crystal stars, and fails.
And Bowser is a playable character in many games. You can be him in the Mario Kart series, SSBM, Paper Mario: The Thousand-Year Door, and I think in Toadstool Tour.
Below are the only games in which Bowser poses as a villain that kidnaps Peach:
Super Mario Bros, Super Mario Bros 3, Super Mario World, Super Mario 64 (and DS version), Sunshine, and Paper Mario.
I think about it, that's not really a lot of games that Bowser kidnaps Peach in.