Webkit would effectively become the engine and standards body which everyone adhered to. IMO, this makes far more sense than the current mess we're stuck with. The standards bodies are ineffective and molasses slow at getting anything done. The distinct differences and vendor-specific features between modern rendering engines demonstrates how irrelevant these organisations really are.
There's room in our world for plenty of different browsers and interpretations for what the browsing experience should be, but using a different engine should not be optional for anyone expecting mass acceptance.
They'll be stuck hacking on Webkit if they want anyone to really use their product. The world doesn't need another Presto or Gecko. If their changes are worthwhile, it would become integrated within webkit or used for subsequent versions.
In a mono-culture you have no predators thus no evolution.
There's a big difference between a standard and an implementation. If you only have one implementation (e.g. WebKit, or more specifically, WebCore), developers will undoubtedly start depending on certain quirks of that implementation. You reach a point where these quirks can't be changed, otherwise half the web will stop working. Having multiple rendering engines means these quirks are more likely to be fixed, and not relied upon.
Now, everyone switching to WebKit probably wouldn't matter much in the short term, but that seems awfully short-sighted. We don't need a single rendering engine, but new standards bodies. I'd be perfectly okay with WHATWG stepping into this role. What you're describing is "innovation" in its weakest form. It's basically incremental improvement, rather than real innovation, and I can't help but think that's the wrong approach.
Real innovation is far more drastic and disruptive. When it comes along, it often meets heavy resistance from those who are entrenched, especially when these people may not benefit from it. This is true within open source projects too. Unorthodox ideas will often be brutally crushed, even if the benefits they provide are immense. It's impossible to say if WebKit will continue in a direction most agree with in 5-10 years. Sure, one could always fork if that happens, but then how do you get that into the hands of your users? Needless to say, it isn't a compelling argument for a single rendering engine either.
There's other reasons to be concerned with WebKit becoming the defacto implementation, as well, that have more to do with its architecture, and not simply the rendering engine. For example, if you want parallelism, WebKit is a non-starter (too entangled, too much code making too many assumptions). Also, nothing like Mozilla's Servo project could happen.
I'm still not seeing how a single, dominant rendering engine (run by an open-source consortium) could ever be a bad thing.
I'll agree that
if there is to be a rendering engine mono-culture, it might as well be open source, but that in itself guarantees nothing. Apple still has quite a bit of control over the project, and it's mostly developed by Apple, Google and RIM (full disclaimer: I work for one of those companies). There's nothing preventing them from getting a bit heavy-handed just because it's open source.
In more serious terms, having more hands on the same code must be good for WebKit as a whole - more developers hopefully means more bugs squashed, and possibly better standards compliance (and fun new toys 
