This question deserves a more thorough reply. I understand that, having lost (via a machine upgrade) the keychain persisted iPhone backup password, that I cannot access the existing backups. Fine. I just want to start over. However, it appears that the password is encoded in the iOS device - removing your existing device backups via the Preferences pane in iTunes does not allow you to start again with a new password. When you plug the device in afterwards, it still has the "Encrypt Backup" option set.
In effect, this means that if you lose your backup password, you can not only never restore from your existing backups, but you can never create new ones from which you could.
This is clearly not well thought out and is a somewhat shocking UX failure from a company that, to my understanding, still somewhat prides itself on its abilities there. I have biometric/passcode unlock access to the physical iOS device, and login access to the macOS instance running iTunes. There is nothing more to protect. iOS should offer a way to drop the existing backup password on the device so that I can start over with a clean backup.
I will add that this entire "encrypted backup" story is poorly thought out. I'm backing up to a laptop that has an encrypted filesystem. I shouldn't need another password, and I shouldn't need to choose between backing up all of my data with yet another password, or having a partial backup (no passwords, no healthkit, etc.) that drops all of my creds. Everything I'm backing up is going to an encrypted file system anyway, and surely iTunes could know that.
To my way of looking at it, by providing no obvious means of dealing with a lost iOS backup encryption password, Apple has "soft bricked" my device. If I lose it, I cannot recover anything.