Hi, Mithril author here.
> With IDE support being so good why would I ever subject myself to using non-html view code anymore
As others have mentioned, preference for angled brackets can be solved with tooling. insin's MSX exists precisely to address this very concern, and it's dead simple to setup. As you might have noticed, Mithril's syntax is designed to look like the other language of the web: CSS. In practical terms, what this meant to me was to replace the usage of Emmett CSS-to-HTML expansion macro shortcuts in my text editor w/ a macro shortcut that spits out `m("")` instead. Coupled with simple IDE features like automatic bracket matching, I think you can get a typing experience that is very close to what is familiar to users of productivity tools like Jade/Haml/etc users (and with no preprocessor step required).
The actual reasons for templates to be pure js are slightly more technical: it gives better error reporting, proper scope/closure/variable shadowing rules, turing completeness (for the purposes of refactoring/abstraction, etc), it's friendlier to transpiled language users, it's lintable, it's compressable, it's statically analizable, etc.
Also, in my experience, complex apps tend to get heavier on logic than actual markup (this was actually the first thing my previous boss noticed when I gave him a simple code example when I was showing him the framework)