Speed and nice error messages in (La)TeX, and to some extent ease of programming, are entirely doomed because of fundamental design choices. Being based on unhygienic macro expansion means that there is only one way to evaluate (the slow way), there will never be incremental compilation (everything can possibly be stateful in horrible ways), there will never be good error messages because there's basically no AST information anywhere (begin/end is a joke).
Regarding ecosystem: tons of undecipherable LaTeX packages are basically one-liners (ok, 10 liners) in typst. I know it from experience: I've written my PhD manuscript in typst. So perhaps one reason why there are so many (basically frozen) packages in LaTeX is because they are so hard to write and maintain.
edit: of course, being only a few years old, typst is nowhere near as solid as TeX, but you can already use it for a lot of things and its a breeze to use.
Regarding ecosystem: tons of undecipherable LaTeX packages are basically one-liners (ok, 10 liners) in typst. I know it from experience: I've written my PhD manuscript in typst. So perhaps one reason why there are so many (basically frozen) packages in LaTeX is because they are so hard to write and maintain.
edit: of course, being only a few years old, typst is nowhere near as solid as TeX, but you can already use it for a lot of things and its a breeze to use.