I agree that 64-bit is likely to be a very long-running standard. But, given that on 64-bit, there's no agreement on sizeof(long) - and it's impossible to change at this point because it would be a massive breaking ABI change for any of the relevant platforms - the only sensible standardization approach for C now is to deprecate short/int/long altogether and always use int16_t/int32_t/int64_t.
It helps to look at the history of C integer type names and its context. Its origin lies not with C, but with ALGOL-68 - as the name implies, this is a language that was standardized in 1968, although the process began shortly after ALGOL-60 Report was published. That is, it hails from that very point in history when even 8-bit bytes weren't really standard yet nor even the most widespread - indeed, even the notion of storing numbers in binary wasn't standard (lots of things still used BCD as their native encoding!). ALGOL-60 only had a single integer type, but ALGOL-68 designers wanted to come up with a facility that could be adapted in a straightforward way to all those varied architectures. So they came up with a scheme they called "sizety", whereby you could append any number of SHORT or LONG modifiers to INT and REAL in conformant code. Implementations could then use as many distinct sequences as they needed to express all of their native types, and beyond that adding more SHORT/LONG would simply be a no-op on that platform. K&R C (1978) adopted a simplified version of this, limiting it to a single "short" or "long" modifier.
Obviously, this arrangement makes sense in a world where platforms vary so widely on one hand, and the very notion of "portable code" beyond basic numeric algorithms is still in its infancy. Much less so 40 years later, though, so the only reason why we still use this naming scheme is backwards-compatibility. Why use it for new code, then, when we had explicitly sized integer types since C99?