Most required upgrades do not involve anything "busted". Weaknesses are often noticed long before any practical attacks are available. If you want to upgrade, say, Wireguard in such a case you would have to switch over the endpoints in pairs. Obviously that is going to be impossible in practice so the system will get backward compatibility grafted on in a fragile and dangerous way.
OpenPGP is an example of a case where relatively extreme backwards compatibility is required as old archived messages have to be accessible. But that isn't a problem because things are such that downgrade attacks are impossible. The list of desired methods is in the public key which is signed with itself. So downgrades are not always an issue.
Do you have an actual example? Normally when people talk about a downgrade attack on OpenPGP they just assume it is somehow possible without actually checking that it is.
Note that I am only claiming that downgrade attacks are technically impossible for OpenPGP due to the way that it works. To break the protection against downgrades means that you have to break the root cryptography. That might not be true for other stuff... Makes for a great example though...
OpenPGP is an example of a case where relatively extreme backwards compatibility is required as old archived messages have to be accessible. But that isn't a problem because things are such that downgrade attacks are impossible. The list of desired methods is in the public key which is signed with itself. So downgrades are not always an issue.