That's because they see older versions of Android decrease in usage so they think it's fine to lock them out and potentially lose customers[1], but they're not going to do that to the majority of them.

If the majority stops falling for the propaganda and "upgrading" to a worse experience, other businesses will follow.

[1] I have told businesses that changes to their site have made me no longer want to do business with them, and seen responses ranging from complete dismissal to quick reversion.