Every year FileMaker launches a new version in order to meet the new development or just a marketing gimmick to grab more customers. It looks like a money maker machine for many old users. So many years we need to pay upgrade fee for the new version. If you skipped the upgrade version then you have to buy a new version in full price such as FileMaker Pro 16 upgrade version is not available for 13 or lower version.
I suggest FileMaker creating a membership version for individual users. The member can use FileMaker new version as long as paying monthly or yearly membership.