99.9% users will upgrade to gpg2.1 once and never think about downgrading. On Debian for example, to get gpg 1 or 2.0 back, you'd need to rollback the whole dist-upgrade which is so tricky (and officially unsupported) that restoring the whole system from a backup is the only realistic option. Thus, offering to delete secring.gpg on key deletion wouldn't be obnoxious at all. Secret key deletion already asks a number of questions, one more wouldn't be bad. I guess deleting secring.gpg when the passphrase is being changed would be a good idea, too.

The migration documentation is something distro maintainers read, but it's humanly impossible for an user to read all such documentation on every of several thousand packages on a dist-upgrade.