Most software companies that don’t offer subscriptions do it the same way. But most at lease offer a so called grace period. When you buy some software within x months before the release you get the upcoming version for free.
That’s what Counterpath should have done. They obviously have not, have they?
What still can happen is that you buy software just before such a grace period starts. What do you do then? You see, the software is quite hard when it comes to upgrades/updates.