My understanding is that KDE4 was itself the result of a significant refactoring. My guess is that it's only a matter of time before the conversion happens (and by the looks of it, the devs are already looking into it).

A lot of things in KDE4 were (re)written from scratch. No such thing will happen for KDE5. It is mostly about splitting the base libraries.

That said, Plasma and KWin will probably have the most changes, one from the mass conversion to QtQuick, the other for supporting Wayland.

Any KDE based application could in theory start using qtquick instead of qt C++. I don't know why you would take a C++ implementation and make it run on qtscript, though.

I easily see new applications written against kde5 using a lot of qtquick and js implementations. Especially with Ubuntu phone coming, there would be little to change between some gui designs between supporting the desktop and phone.