It's so much more pernicious than that. As long as one company controls the One Rendering Engine to Rule Them All, one company de facto controls the Web itself, and everyone else is mostly just along for the ride.
As long as one company controls the One Rendering Engine, all websites are tested against that One Rendering Engine (i.e. not the truth of the spec).
It becomes impossible to create a new rendering engine without actively copying the quirks behaviours.
Sadly, I fear that the reason Free Software folks don't seem to be getting quite so keyed up about Chrome is that, at least for now, the One Rendering Engine happens to work on Linux. It seems that even principles can be mercenary.