This isn't the first time this issue has come up. WordPress 2.0 was meant to be a long term stable version, but support for that ended up being dropped before it was even officially stated until. At that time, it appeared (to me) that there would be no backported security fixes available in future at all.

If the team wants to continue providing this backwards compatibility, then perhaps it should just be stipulated that it's only for one or two point releases backwards. If someone can't upgrade in six months, then they probably never will. You can't keep backporting fixes eternally; it's an ever growing number of versions to support.