It must be possible to keep the current structure (some form of a DOM plus a scripting language with a full access to it), and still have a dead simple Tk-like

It must be possible to keep the current structure (some form of a DOM plus a scripting language with a full access to it), and still have a dead simple Tk-like event model around it. It's all about overengineering. Tk demonstrated that all this stuff is fundamentally easy. But the modern web technologies are overengineered to such an extend that anyone touching them draw back in fear. So, yes, now the only way to put all that complexity under control is exactly this - things that compile to js (+ css + html), that can hide the overengineered parts of the underlying ecosystem and only expose simple, obvious interfaces. PureScript is a right thing.