> Say, for sake of argument, that I'm the World's Greatest Widget Engineer.
Say, I am not.
> so I can get stuff I want by negotiating individually with my employer
I cannot, see above. Does that mean I don't deserve to have the bargaining power for the best deal for myself?
> A union's interests are probably not completely aligned with mine—they're focused on protecting the majority of employees, who are probably not as valuable as me—so forming a union could very easily lead to me getting less of the things I want. For example, if a union convinced a company to pay/promote based on seniority rather than performance, that could be good for most of the people in the union but bad for me.
Well, based on the fact that I'm part of the majority i.e. not as valuable as you, it works very well for us (who are not the World's Greatest Widget Engineer).