A Blink monoculture destroying "the web", like the one we almost already have.
Having a single renderer but multiple browsers on top of it would vastly simplify the web for users & developers, resulting in one consistent engine to develop, test, and fuzz against.
Rather than being the irrelevance it is today, Mozilla focusing on Blink development & governance would have way more impact for billions of users, rather than some obsolete and forgotten build of Firefox.
Then all it'd take is for something like the E.U. to cripple Apple's Webkit-on-iOS monopoly for Blink to take over there too.
Having multiple renderers is just a waste of everyone's time...
Unfortunately, Blink isn't openly governed. It's governed by Google, and if Google decides that they won't implement complex feature, maintaining that feature will be logistical nightmare. Imagine writing drivers for some exotic kernel, but kernel API or even very architecture isn't stable.