Ragarding your edit: A startup called famo.us(http://famo.us) is focusing on exactly this (canvas based UIs) however I'm not completely sold that this is the future. It's an awful lot of wheel reinventing.
They're doing it in the name of solving performance for HTML5 apps and for creating apps with rich UIs which would be more similar to interfaces we see today in games rather than traditional DOM based UIs. However, I don't think there's a need for that, and ditching all HTML standards in order to reimplement them in the canvas sounds like a step backward for me.
I need it specifically for games, building an interface on a canvas without even layout managers or mouse click aware components feels like the stone age, so I am in the market for a wheel actually. Though it does seem like a bit of a waste as the other poster also said.
That works fine to an extent, but sometimes you want your menus integrated with your game and have consistent graphics and animations and so on. I used dom elements for snaketron, but in hindsight I think it felt tacky (it might partly have been because I used Bootstrap). For most games, which are more complicated, I can imagine it would be a pain to maintain dom menus.
FYI, famo.us is not Canvas, it's DOM. IMHO, since AJAX and div-hell designs became commonplace, we've been needing to have a long overdue discussion about the right role of semantics in HTML. There definitely is a place, the <article> for example was a step in the right direction because it indirectly acknowledges that an html document is not really any longer a document, but in many cases is simply a collection of documents. Take Twitter for example. Each tweet is a document, but is twitter itself a document? no, it's an app. We still need standards and semantics and all the goodness of the DOM, but we need it for things on the web that are in fact still documents and not some chimera.
At famo.us, we're focusing on turning the top level DOM elements into something compatible with the app approach that is capable of being made performant. However, we're not going to throw the baby out with the bathwater. DOM and the original ideas behind HTML are still valuable. It's just time to reassess the role of apps and how to make it so they can live in the same space as documents (i.e. the browser viewport), especially when they are ephemeral apps, because the "install" is dead. It's been usurped by the almighty hyperlink.
They're doing it in the name of solving performance for HTML5 apps and for creating apps with rich UIs which would be more similar to interfaces we see today in games rather than traditional DOM based UIs. However, I don't think there's a need for that, and ditching all HTML standards in order to reimplement them in the canvas sounds like a step backward for me.