The problem is not finding limits, although that does take decent engineering for anything non-trivial. The problem is that a hard limit assumes your requests have a relatively uniform distribution of memory requirements. Otherwise, you are wasting memory. Or you have N limits instead, which just complicates the problem.

Resize: realloc invalidates pointers. If you're saying allocate more chunks and link them, that is technically no longer an arena?

Requesting virtual memory and committing lazily seems a good trade-off except on Windows, which does not allow to over-commit. Somehow I don't find that very satisfying because it relies on the OS and on the hardware to provide virtual memory support. But maybe it's just what it is.