It's mostly that I'm quite personally invested in TeX, far more than most people are. I work on it nearly every day, and I'm even hosting an international TeX conference about a month from now [0].
I do have a few minor concerns about Typst, mainly that its parsing/compiling strategy allows less introspection and extensibility than TeX does (but this is necessary for its fast compile times), and the fact that it's backed by a for-profit company seems less robust than TeX's 50-year history of volunteer maintainership. And of course Typst is still missing quite a few features, but if it's successful I can't imagine that this will be an issue.
Also, I personally mostly use ConTeXt [1], which compiles much faster than LaTeX and has a very uniform syntax, so switching to Typst wouldn't have as many benefits compared to if I were switching from LaTeX.
So if Typst were to take over, I'd mainly be disappointed that my very deep knowledge of TeX would be no longer relevant. But I also wouldn't be surprised if Typst ultimately revitalized TeX, since it might motivate us to improve TeX even more, and Typst might be a good entrypoint for users interested in plain text typesetting (who might then switch to TeX if they find Typst too limiting).
[0]: https://tug.org/tug2026/
[1]: ConTeXt uses the same underlying TeX engine as LaTeX, but different macros. See https://tug.org/TUGboat/tb46-3/tb144berry-engines-formats.pd... for a brief overview.