I've bought full versions of Kontakt 2, 3 and 4. When Kontakt 5 came out it offered no new features that interested me and it bundled the exact same factory content as K4. It didn't seem like a worthwhile purchase.
Apparently, I wasn't the only one who looked at K5 and said "meh", because K5 was initially not the big seller previous major releases had been. You can't just add a couple obscure features that don't apply to most users/developers and expect everyone to hand money to you just because it's that time again.
Native Instruments, however, goes out of their way to make sure you pay a penalty for not re-upping. They accomplish this through
intentional version incompatibilities. Libraries created under version 5 cannot be loaded into any previous version. They even broke compatibility between minor fix releases, such as going from 4.2.1 to 4.2.3.
After K5's release, in order to maximize their potential markets, established third-party vendors of
non-licensed libraries continued to release under K3.5 or K4, which K5 can load. They could thus refuse to play NI's game. Unless they specifically needed a feature unique to K5, there was no reason to create any new libraries under it that would then be inaccessible to the majority of Kontakt users.
New library developers, though, do not have this option. If you don't already have K3.5 or K4.2 you can't buy them, so latecomers have no choice but to build K5-only libraries. Kontakt does not give you the option of saving instruments that are compatible with previous versions, so a developer who only has K5 must create K5-only content.
Imagine if MS Word 2013 could only save Word 2013 files that could only be read by Word 2013, making it impossible to send a document to anyone who was running anything older. Great way to coerce the world into bi-annual re-purchases, but even mighty Microsoft would not be so arrogant.