Maybe you misunderstood me? I wrote "it doesn't lead itself to easy incorporation of features" (such as the above). I also acknowledge several attempts to fix this. What I lament is that "We're stuck with a codebase that works, but is messy, too complicated and obsolete in many parts".

I know a lot of the above are supported (I use LaTeX myself, and follow the developments quite closely) in newer TeX implementations, but not in clean way and not with first class support, to the legacy limitation.

I was asking for a clean implementation, no something like XeTex --that while supports several modern features it inherits tons of design problems from the original TeX.