To maximize fork divergence, it might make sense to require this. Any
sensible proposal for a hard fork would include a change to the sighash
anyway, so might as well make it required, no?
Compatibility with existing transaction-signing software and hardware should be considered.
I think any hard fork proposal should support a reasonable number of reasonable-size old-sighash transactions, to allow a smooth transaction of wallet software and hardware and to support anybody who might have a hardware wallet locked away in a safe deposit box for years.