There is a cost for every major version upgrade, so when they eventually (in about a year or so) go to version 21 there will be an upgrade cost at that time if you wish to go to v21. They usually do discounted upgrade pricing right before/after release with a list of changes planned for that major version.
There is no requirement to upgrade. In the past, I have gone 2-3 major releases before seeing a need to upgrade (based on my usage at the time and feature changes). Since about v16 (for the PC), and since v18 for the Mac, I have updated both licenses every major release thanks to the new features and ease of use changes.