You only pay for major updates if you choose to upgrade. For example from v1 to v2, from v2 to v3 and from v3 to v4 ...etc etc etc
but you dont have to pay for the incremental upgrades for example v3.5.4 to v3.5.5 , because you are technically within the main version.
This is standard practice within music software, specifically music DAWs.
Also note, every DAW company including PreSonus have discounted prices for users who already own a previous version of the DAW.
prices though depend on the company, some companies go as far as 75% off and some have 50% off.
I have yet to find a company that goes lower than 50% for its customers who already own the DAW product.
Hope that helps :)