> When a project proudly declares itself as "modern," it implies that it's up-to-date and built with the latest technologies.
> Would anyone suggest replacing TeX with a "modern" alternative simply because it's newer?
No, “modern” implies that the existing solutions have some shortcomings which are perceived to be due to old age or legacy/tech debt.
That’s why no one is proposing an alternative to TeX.
Whether that perception of old age therefore bad is valid or not is a different question. Chances are that the “modern” solution will end up reinventing the wheel and rediscovering why the old tools did things a particular way.
Other times modern means the new tool cherry-pick the best part of its predecessors and omit the bad parts.