I'm not defending anything, I'm saying it's usually trivial to make allocation time marginal.

+ every application doing manual memory pools

Usually it's simple data structures in flat memory.

If allocation is taking all the time, that's a poorly optimized program with lots of low hanging fruit and a different allocator is not the right fix. It's like having a boat with a hole in the bottom and someone says the solution is a smaller hole.

But TCmalloc / jemalloc are 21-22 years old, respectively, and jemalloc has been the FreeBSD (released) default malloc implementation for the last 18.

jemalloc is also possibly bigger than all of musl. If it was a problem after optimization I would use it and I have in the past, it's just nowhere near as important as minimizing allocations in the first place. OpenBSD uses straight mmap.