We support a change to the version bits of the HD serialization that will inform the receiving utility of the exact derivation method used for the pubkeys. Third-parties handling xpubs must not require additional information from the user about the derivation path or serialization format of the addresses under that xpub. When you have to ask, "Is this a SegWit xpub?" then you've already lost.
Avoiding a total UX nightmare is in everyone's interests.
I think Luke and Thomas may be talking past one another. When exporting a root master HD seed, encoding the {x,y,z}{pub,prv} distinctions makes no sense, as the root seed should derive all paths for all coins. Wallets may need additional code to discover which paths have been used when importing a root seed. But when exporting / importing an account-level seed for watch-only and receive address generation, changing the serialization version bytes is appropriate and (in our view) essential to avoid loss of funds.
The Electrum approach is nice but may not go far enough, as xpub and zpub both list "P2PKH or P2SH." Why not expand the number of version prefixes to eliminate the ambiguity?