The issue here is not rapid adoption, it's an effective monopoly being used to lock competitors out of the marketplace by making fast, arbitrary changes and forcing people to keep up. This has already happened in a few scenarios despite WebKit having competition, so it will get worse if all the competition slowly dies off. webkit-only mobile websites are an obvious example but chrome-only audio code in HTML5 games is another great example (Chrome's HTML5 audio stack was so broken it caused crashes, so to have working sound in Chrome you had to use their special API. As a result, there are HTML5 games out there that only support Chrome's API or, if you're lucky, have a fallback based on a flash plugin).
EDIT: Another great example is the stunt Intel pulled with AVX to intentionally sabotage AMD's ability to compete in the market, as documented here:
http://www.agner.org/optimize/blog/read.php?i=25
Essentially, Intel published a proposed new instruction format, and AMD said 'that looks great, we'll be compatible with it'. After AMD announced this and had started preparing to ship their new chips, Intel suddenly announced that they had changed their instruction format from what they published - after it was too late for AMD to adapt.
The end result was AMD shipping chips that were incompatible with Intel's despite AMD's best effort. Intel knew that as the majority market share holder, developers would prioritize Intel compatibility over AMD compatibility, and AMD would lose.
EDIT: Another great example is the stunt Intel pulled with AVX to intentionally sabotage AMD's ability to compete in the market, as documented here: http://www.agner.org/optimize/blog/read.php?i=25
Essentially, Intel published a proposed new instruction format, and AMD said 'that looks great, we'll be compatible with it'. After AMD announced this and had started preparing to ship their new chips, Intel suddenly announced that they had changed their instruction format from what they published - after it was too late for AMD to adapt. The end result was AMD shipping chips that were incompatible with Intel's despite AMD's best effort. Intel knew that as the majority market share holder, developers would prioritize Intel compatibility over AMD compatibility, and AMD would lose.