Enable sighash default option for non-taproot(or tapscript) spend? #29857

Why does not enable sighash default for non-taproot transaction(i.e. p2pkh, p2sh, p2wpkh, p2wsh)? I think it's great BIP341 adds sighash default to save a single byte, which could be huge in aggregate. As segwit v0 and legacy transactions still take a lot of portion, wouldn't it be good to enable sighash default for them? It's more efficient, and increases overall backward-compatiblity(without concern about malleability in my humble opinion).

That's not possible without a hardforking consensus change, which would need extensive discussion and widespread agreement among the community.

Furthermore, anyone who would be interested in the byte savings enabled by such a change could instead just migrate to using taproot, which would result in far larger savings for most users.