Now whether the amount of work involved would be worth it, that's another matter.
Also sort of a reply to Ashley: It is if you want console and desktop export, and technically also mobile export since why would you use HTML5 on mobile when you can just export real Android and iOS apps??
As for "A closed source game engine which doesn't care about us at all", I think there's three major issues with that way of thinking:
1. Chrome/FireFox/whatever is for all intents and purposes a web browser, the ability to render interactive content is something they're improving (SLOWLY) due to HTML5 + WebGL being included in the web standards, but game engines inside HTML5 are not their main focus, and for anyone following the Chrome jank issue they can see that it's been going on for years. We have been lucky with some fixes so far, but I don't think we can assume that anyone on their end really specifically cares about making C2/C3/our exported games work in their browsers. Ashley has been doing a great job at pushing for fixes and I appreciate that, I love Construct and I wouldn't argue if I didn't care about its future.
2. If Unity was a tiny obscure company I'd mind the closed-source part, but when is Scirra going to get their hands dirty in the source code of these open source browsers and make their own player app that's optimized just for Construct games? That's what I'd need to see to believe that a company which hires hundreds of employees to make a game engine is going to be beaten by a small middleware company with a much smaller customer base.
Sure, they might make drastic changes that require re-making the plugin, but since C3 so far is just a re-skin of the C2 runtime what's the problem with making new editor interfaces every few years and re-selling? Might as well make it a yearly subscription eh! (and I'd actually pay for it, for real, because Unity is not designed for 2D and it still feels like an add-on, plus events are just nice and I'd feel much more comfortable using them if they exported to some C#/.NET bytecode underneath )