"The biggest downside is probably a long-term ABI problem. The scheme is inflexible. If your Pascal string-length header is too short, however do you expand it? It's wired into every string-using program out there! At least nul-terminated strings need no expansion."
You'd naturally use 32 bits on 32 bit systems for string length. And by a strange coincidence, that's the maximum amount of contiguous RAM that you can address on 32-bit systems. On 64-bit systems, you'd naturally use 64-bit counter.