I do see an advantage to tie it to a time period for the customer. If it was just tied to a major version then Unraid would have to make a decision about if a feature belongs in version 8 or version 9. Essentially locking that feature behind a paid upgrade. By making the upgrade tied to a time period they have no incentive to hold features back for the next major version.
I learned about this book written by Adam who founded the SeaGL conference.
https://selfhostbook.com/
I’ve been self hosting for years and bought it to support him. I like the style of writing and how he explains the concepts.