My own opinion is that if a person purchases Ultimate / Pro very shortly before the Next Release is made available, they really should get the newest release since it is a near thing. This is my opinion and I have no control over the funding of Zorin OS.
Aside from that...
Developers should be paid for their work. Zorin OS does not include third party advertising, spyware, target market advertising or any of the other shady practices that the Big Companies do.
If you bought a 2018 Mazda Miata... Can you expect to get the Next release of the model for free just because you bought a car once?
If your employer refused to give you this weeks wages because he already paid you once last week, would you accept that as valid?
Zorin OS is funded by donations from Zorin OS Ultimate / Pro. Not by ads or spyware. That is what keeps the project funded, alive and moving forward. It is thanks to people like you both who chose Ultimate.
Each release is a New Release, not an extension of the previous one. Some users even opt to skip between; They will choose Zorin OS Pro one release, then go with Core the next after that one, then back to Pro for the Following... and so forth.
But Yes, users must pay for each new Zorin OS Pro release to compensate the Two Developers that devote their time, skill and knowledge into crafting it over the course of the previous year and a half or more.