Thanks for the detailed response.
>compatibility issues in LaTeX often come from packages that redefine the same macros in incompatible ways
Absolutely. A related but more subtle problem occurs when macro A ordinarily calls a macro B, but a package redefines A not to call B at all (perhaps reimplementing part of B itself), and then, when the user includes a second package that redefines B, this latter package appears to have no effect.
Based on your explanation of Typst having a global namespace, I would expect such conventions (like a function A that by convention calls a function B, both of which can be redefined by any package) to arise in Typst just as they have done in LaTeX. (This risk would be much reduced if Typst didn't have functions as first-class values, but from the TFA, I see that it does.)
>Compare to LaTeX where sensible code is broken when a package makes a small changes somewhere deep in the macro pile of cards.
LaTeX has grown "a macro pile of cards" because (a) the base LaTeX system was not comprehensive/expressive enough to let users express everything they wanted to do "within the system" (i.e., by merely twiddling existing knobs in a composable way) and (b) because it is possible (indeed, relatively straightforward, at least initially) to make them.
Maybe Typst has a much more comprehensive and well-designed set of knobs, in which case the conditions will not arise that encourage the same "macro pile of cards" to form. Otherwise, I don't see any reason to expect that it will wind up any different.