1. When I pick a low-level language, that's precisely the kind of control I'm after (otherwise, why go for a low-level language?).
2. Heap allocation is a very common cause of serious performance problems in low-level programs because malloc/free overhead is so high. Some languages have a moving GC to reduce the high overhead of heap allocation, but low-level languages typically can't use that optimisation, so you have to resort to arenas (which offer a similar optimisation to that of moving collectors). This isn't always faster than malloc/free, but it usually is, and if you want malloc/free you can pick that.
Anyway, I wouldn't say that the default in Zig is to use an arena, rather it's to give you the control that's the whole purpose of low-level languages.
2. Heap allocation is a very common cause of serious performance problems in low-level programs because malloc/free overhead is so high. Some languages have a moving GC to reduce the high overhead of heap allocation, but low-level languages typically can't use that optimisation, so you have to resort to arenas (which offer a similar optimisation to that of moving collectors). This isn't always faster than malloc/free, but it usually is, and if you want malloc/free you can pick that.
Anyway, I wouldn't say that the default in Zig is to use an arena, rather it's to give you the control that's the whole purpose of low-level languages.