>Someone else's battle-hardened allocator might be slow or buggy, so you write your own as part of the business logic implementation?

In infrastructure where speed and reliability are highly valued? Absolutely. The gains obtained from proper memory layout and specialized use are massive. As long as you have the reason to do it, it's an easy win. I believe that the Zig standard library has different specialized allocators, so you don't even have to write your own buggy implementation.