The reason TeX is written in a 1984 dialect of Pascal is that the typesetting bugs have been solved in a completely specified language; it is much easier to write a transpiler for Pascal->C than to rewrite TeX. Asking an LLM to rewrite it in the language-du-jour is a huge cost for very little benefit.
BTW it has been so depressing in the last few months to see LLM-generated projects make claims about performance/accuracy, but there is no benchmarking code on Github and the "thousands of tests" are all useless happy paths. I am sure we will see some grifter claim that Claude rewrote TeX and I am sure dozens of credulous HN users will take it seriously. But we won't see a useful rewrite. It'll be resume-oriented slop like that dishonest Mathematica-in-Rust project we saw last week.