This view is far too simplistic, and I’m not talking about binary political divisions at all. The point is that the process for resolving content controversies is a political process, as in they are resolved according to (or at least influenced by) the status of the disputing parties within the social hierarchy of Wikipedia editors. There is no “invisible hand” to guide fairness here, and the potential topics of controversy don’t exist along some sort of binary paradigm of potentially opposing views. I also have no idea why you would presume that any controversial topic (let alone all or most of them) would have opposing advocates on equal footing with each other.

> I think this has been borne out in practise for the most part.

That very much depends on the subject matter. Subjects that are at all politically or socially controversial are very likely to be under the control of one or the other side. If it's a "boring" subject that lots of people are interested in, then the page will probably be fairly balanced.