I do want to do a runtime overhaul (possibly even a rewrite) eventually, but dropping backwards compatibility is a huge problem for a lot of users. I wouldn't do that lightly.
We have a good way of upgrading features by replacing them already: deprecating existing plugins and replacing them. This is exactly what we did with WebStorage. WebStorage got replaced by the new LocalStorage plugin which is different and not backwards compatible, but WebStorage is still supported for existing projects. New projects can only use LocalStorage. So it's the best of both worlds: nobody's existing projects are cut off, and in the long term it moves towards exclusive usage of the new plugin.
Hopefully we can use a similar approach to upgrade much of the runtime features. A clean break might be required if we want to rewrite the runtime core, but hopefully we can get a long way without that.