There may be more people who want a TeX-style layout engine in an embeddable form, without the TeX macro language. Or a multiple-scale version of it, which would make it easier to avoid widows and orphans, by making it possible to reconsider earlier page breaks. (Once upon a time, large computers really could hold only one typeset page in memory; no longer.)

If you who want something 100% compatible with TeX, you already have it, and it's not clear why you'd care about the implementation language.