As I'm writing this there's a big list of core theme functions and templates that are being converted to Twig templates. D8 will (hopefully) be going live with working Twig templates for all the markup it renders.
There's some performance issues around the Twig engine that need to be nutted out and it looks like the smallest Twig templates that have just one or two tags in them will be completely replaced with something else outside the theme system that's faster, but this should all be resolved sooner or later.
Until D8 ships we won't know what's a Twig template, what's been removed from the theme system and what's been converted to something else. Once it ships I think that in D9 there's a good opportunity for us to speed things up a little by taking every Twig template in core, writing an equivalent theme function for it that works with the existing preprocess function and renaming all the twig templates from "foo.html.twig" to "foo.html.twig.example" so they aren't called.
Because it will make everything that little bit faster to run "out of the box" and won't make life any harder for themers - they can continue to override whatever they want with templates and have solid working examples in core to reference (unless D8 ships with broken markup).