Would it be worthwhile to make a fork over HTML imports? Suppose that in a mono-engine world Google decides to build AMP directly into the browser rather than on top of Javascript -- would that be a big enough issue to make a fork? I was (like many people) fairly disappointed about Web DRM, but I was happy that Firefox was at least planning to put some additional sandboxing above what Chromium was doing. Would they have forked a codebase just to introduce that extra sandboxing?
You're right that Open Source does allow us to reverse course and fork Blink later if something terrible happens. Remember though, the moment that someone like Mozilla or Microsoft makes a fork, at that point they're back to the same situation that they're in right now -- even worse, because they're stuck using a codebase that wasn't architectured specifically to meet their goals. That provides a big incentive to wait as long as possible before deciding, "heck it, we'll build our own engine."
What I would suggest is that there are at this moment already at least minor organizational issues within Chromium, and there are currently benefits to having multiple browser engines. We could give up those benefits and get them back later if stuff got really bad. But it would mean waiting until things got really bad; and ignoring all of the tiny, daily decisions before that point that go in to setting the direction of the web.
Remember that the web isn't supposed to break backwards compatibility. If you don't get things right the first time, there isn't a second chance to change them.
And actually it is better for innovation not to use to Blink. WebRender is potentially much more performant than any other engine and it would never had existed if Mozilla were just aligning with Blink
It's not like the other Blink or Chrome based browsers are doing great, sure Brave and Vivaldi is getting some press, but they have even less users than Firefox.
Sadly Firefox isn't peoples mind anymore when they think browser. It's IE all over again, but with Chrome, Only Chrome is moving forward. I don't know if there's anything Firefox can do, other than marketing. It's currently the best browser out there, but that was the case when IE6 dominated the web as well.
Firefox doesn't need to dominate, but hitting some like 20% market share would be fantastic.
This isn't directly relevant in the larger context of this thread, but Firefox's sharp drop—even after the introduction of Quantum, which I've found to be a big improvement—is what makes their situation much more scary to me.
This matters because, on-the-whole, Mozilla's dev teams tend to be much better at writing standards than Google's are; partially because many of the standards Chromium focuses on are prioritized and developed with the intention of being helpful to Google's long-term strategies. There have been multiple instances (html imports spring to mind) where bad decisions were averted purely because Mozilla had the guts to say, "we're not putting that in Firefox, come back when you have a better proposal."
Yes, Chromium is Open Source, but Open Source isn't magic. It's still very much a Google product, designed to increase Google's overall influence over the direction of the web. Unless Mozilla's plan is to literally fork Blink, then having a primarily for-profit mono-culture in charge of what features get fast-tracked and prioritized for browsers is still incredibly harmful.