yes but that is already true today, and it's wrong. you should not sell bug fixes and security patches. anything you sell needs to have value. minor upgrades do not have enough value. i am certainly would not pay for a minor upgrade. i'd just wait until enough new features have accumulated until the upgrade is worth it.
so charging for minor upgrades means that you end up with having to support users who didn't upgrade. that makes your support more expensive.
realistically i believe you don't want more than 2, maybe 3 versions of your product in active use, to keep your support load under control.
you could force people to buy minor upgrades by refusing to support the old version, but that would come across as exhortative.
Never trust any contract of the form "X is free unless we say it's not"