I understand your frustration, at least a little, but I don’t see it the same way. This is probably what the founder of The Soulmen meant, when talking about users expecting constant updates. (He was also wrong why software subscriptions are good, but that’s another story.)
Counterpart released an update (Bria v5) in June which works with Sierra and is also compatible with High Sierra. That means you can update your OS to High Sierra and the current version of Bria will continue to work. I’d say this is the usual way. You don’t have to upgrade Bria because of High Sierra but you can upgrade Sierra to High Sierra without having to upgrade Bria, because the current version (v5)is already compatible.
What you cannot expect in my opinion is from developers to maintain older, legacy versions to be compatible to new OS versions, especially if the current version was released a fair amount of time before the new OS version. Because the OS changes from version to version. Sometimes more, sometimes less. And that’s only what you see from the outside. Under the hood it might be even more.
Also if legacy versions of your software won’t work with upgraded OS versions I think there must have gone some effort into the new software version to make the current version working with the new OS, which would justify the upgrade price.
So in my opinion Counterpart’s decision of not starting to work on Bria v4 is understandable.