Actually, they didn't say that the pain of maintaining their own work was too much of a burden, they said the pain of maintaining their model within the constraints and obligations WebKit had was too much of a burden (and, imposed too much of a burden on WebKit with regard to what WebKit wanted to do), so they decided fork so that WebKit can do what it wants to do without worrying about Chrome while Blink does what is needed for Chrome without worrying about WebKit.
> Why can't Chrome implement WebKit 2?
Implementing WebKit 2 wouldn't solve the problem going forward. Changes to WebKit to serve Chrome's needs would still have to meet all of the commitments WebKit has, and Chrome would still be placing demands on WebKit.
> This seems like a failure of open source.
Forking isn't failure. The purpose of open source isn't to create a monoculture, it is to enable groups to share effort when they have common goals, and to allow groups with divergent goals (including where those goals diverged after a period of shared work) to continue to benefit from the shared work without having to start back at square one. This is a success of open source.