Part of the incentive to fork was to avoid having to maintain support for multiple JS engines in the core project. I don't think that means that Blink requires V8 so much as that Blink's support for JS engines (compared to pre-split WebKit's) no longer includes explicit support for the engine used by Safari (whose name I can't remember) as part of the source tree, and presumably will over time be more tightly adapted to the needs of V8.

This would seem to mean it will progressively become more difficult to build a Blink-based browser using a different JS engine.