Problem with this approach is, that it only works for fairly simple stuff. Todays webapps have pretty complex UI with a lot of ramifications and complex dependencies of widgets. Such things are much easier to build if you have a full fledged programming language (javascript) and not only a limited (turing-incomplete) templating language.

So your approach might work for simple apps, more ambitious stuff will get overly complicated.

That is imho one of the strength of mithril.js: Simple things are trivial, complicated things are pretty straight forward to solve.