When purchasing a license for EasyCatalog you’re purchasing for a specific version of InDesign — e.g. InDesign CC 2018, InDesign CC 2019.
With all new licenses you receive twelve months complimentary maintenance; if purchasing an upgrade you receive six months complimentary maintenance. So, if Adobe release another version during this maintenance period the upgrade will be free of charge.
After free maintenance period expires you can either continue on maintenance (which costs 15% of a new license cost per year) or purchase upgrades when necessary. If you continue to use the version of InDesign that you’re licensed for then no further payment is necessary.