I don't see why WebKit-everywhere would be different. If they become lazy and slow at implementing features, what's going to make them get their act together other than a competing engine that's beating them in some regard?
The thing I'd be most worried about with a single engine is lack of compliance with the standard. If multiple people are implementing the standard independently, there are bound to be some differences (for example, if an SVG line is rendered at 0px, with a 1px stroke width and crispEdges, do you snap to [-1px, 0px], or [0px, 1px]?) People run into these issues in practice, and report them; it causes clarifications in the standard to be made, and ensures there is an accepted standard of behavior for pages. This ensures we don't end up in a situation like Ruby, where Specs ran on the official compiler dictate language rules. It means that WebKit is allowed to change its implementation details without anyone crying foul when the maintainers change undefined behavior.
Just because it's open source doesn't mean that the gatekeepers of WebKit will accept your patch.
And while we are on the subject it seems like people are talking past each other in this debate.
1a. Web standards are good because they provide a baseline of combatibility for browsers. 1b. Webkit forks on all browsers could be good because it would provide a baseline of compatibility for all browsers.
2a. Web standards could be good because they prevent any one browser from setting defacto proprietary standards that lock other browsers out of the market. 2b. Webkit on all browsers could be good because it would prevent any browser from setting defacto proprietary standards that lock other browsers out of the market.
3a. Having webkit on all browsers would hurt standards by becoming a defacto standard providing a baseline of compatibility between browsers and still allowing competition. 3b. Having webkit on all browsers would provide a real baseline of compatibility between browsers while still allowing competition.
It's a little bit funny to watch actually.
You don't think it had anything to do with the introduction of Safari and Webkit, more than a year earlier?
Also, the last version of Internet Explorer for Mac was v5.2, which by 2004 was getting pretty long in the tooth. Many Mac users had already switched from IE to other browsers by the time Firefox was released (let alone when it was halfway decent).
Safari was utterly irrelevant until the late 2000s, where it only gained relevance through the iOS devices. It did nothing to spur movement. Further from a history perspective, while Safari might have preceded the coining of the Firefox brand, Firefox was an offshoot of the Mozilla Suite (effectively splitting the browser out of the suite) with a history going back to 1998.