Not all 'nice guys' are wimpy. You can be a nice guy AND be assertive, stand up for yourself, take risks, and take credit for your own work. Heck, you can even be part of a confrontation and still be nice. This IS being nice and perhaps even honest. A person who can't do any of the above is a wimp. wimp!=nice guy.
Now, a 'douchebag' would be a guy that takes credit for others' work, is confrontational, puts everyone else down, and takes risks that put others in jeopardy. Some people like being douchebags, and some people just like douchebags.
You CAN win and be an honest, stand-up guy. You won't if you are a wimp, and douchebags never win, even if it looks like they do.
I've hired wimps, douchehbags, and stand-up people. When the time came to let them go, the douchebags and wimps went first. Stand-up people went last, but they got great references and any assistance I could give them. That, in the end, is winning.