There are recent alternatives that seem pretty ambitious (I'll just mention https://www.tug.org/TUGboat/tb38-1/tb118cozens.pdf as a proof of existence). That I can see, though, any system capable enough to do as much is just as complex. Would be interested in being proved wrong, though.

As other people have mentioned, the LuaTeX folks have got a real good thing going. It is climbing the adoption curve but it makes reasonable some things that in the past have not really been reasonable to program (at least for me).