Gotta think about what "succeed" means. HTML5 isn't doing anything Flash wasn't doing, down to the level where the tools certain people are using might not even change. Anyone who says HTML5 won't succeed is being silly. It's the successor to a very well established use case. How big will it be for gaming? Well, I don't think the Scirra team has anything to worry able (if you're product makes content for the thing that's going to supplant flash, you're golden), but I don't think it's the next big paradise of gaming or whatever, in the same way Flash never quite was. They're tools for certain jobs and design spaces that have pros and cons. Are you even making a game appropriate to be a webgame? Is it appropriate for mobile? If you're just going to put the whole thing into an EXE browser wrapper, is it necessarily the best tool? Even if it's not the necessarily best tool, does it still work for your project scope and your skill set?
You don't just release for every platform for free, even if your platform supports it. Resolution disparities, variation between controls, processor power -- it all matters. You can make a mobile touch app that can be released to all "greater" platforms but it's probably not going to be a particularly enjoyable user experience. I know I took a pass on C2 not because I thought it was bad, or html5 has no future, but because it didn't fit my use case (at least at this point). Plenty of other people are going to find a lot of success there and probably make buttloads of money in that space. :)
Just some perspective.