> OTOH, if an incoming string doesn't have a NUL byte your program will crash/corrupt memory/worse.
how are linked lists different? if the last node contains garbage for its next pointer, the outcome will be exactly the same. it's a bit more rare to encounter an "unterminated" linked list, but I've seen it happen plenty of times deserializing a linked list from disk or if the programmer just forgot to initialize the pointer. c strings basically are linked lists with an implicit next pointer.
If what you say if you want mutable strings, many languages have those and you don't need anything like "NULL" to have them (and you can use a bytearray of the string in Python, though Unicode complicates this).
>The NUL character is in my experience not so terrible, you typically have null pointers at the end of a linked list or whatever as well and nobody complains about that
That's not the same thing at all. The linked list is comprised of structs with next fields, that can be null or point to something. Your program can handle either just fine, as both are valid cases (a linked list expects to find the NULL guard at the end but also expects a non-NULL next pointer if the node is not the last one, so will handle both).
OTOH, if an incoming string doesn't have a NUL byte your program will crash/corrupt memory/worse. On top of that, you need to remember it to add it/make space/for most string manipulations. Strings are not expected NOT to end with NUL, and when they don't there's no way you can mitigate it, except to set arbitrary limits to how many characters you consider.