You may be right that they are going to stay at low initial / full price upgrade... I see two issues with that (for Apple).
One, if the upgrade is not major enough to convince people to shell out for it. This fragments their apps ecosystem, and increases the support headache.
Two, how do you carry people forward to the new version. If it's a separate purchase, that means it's a separate bundle, and won't show up as an update when you check for updates in the mac app store. Again causing confusion and fragmentation (you now have FCPX and FCPX.1 on your machine, which one opens that project file?).
Again, I could be wrong, but Apple's current strategy look unsustainable to me.