Aside from technical problem which already mentioned by @pooya87, who's the target of this security method? I expect that,
1. Regular users don't bother with such complex setup.
2. People with serious security concern already setup their airgapped/cold wallet and don't bother switch to new protocol.
3. Exchange won't find it's useful since they regularly move big amount of Bitcoin (which means they always need "master privatekey" & "master passphrase").