Using DTP since v1 then DTTG as my paperless office with the help of a ScanSnap, I feel the upgrades have been fair, although the latest v3 desktop upgrade was more costly. I’m using it on a daily basis although not using lots of pro functionalities (scripts, wiki, etc.).
Reality is that many developers switch to subscription model because it is the only way they can ensure financial stability. Many application which don’t have (yet) subscription release major upgrades almost every year which, in a way, is similar to subscription. It’s true that we can stick to the previous version… until supported by new OS. Very few still updates previous version just for system compatibility.
Anyway, there are different subscription models and different cost. If the subscription is let’s say, more or less equal in time as the major upgrade, then it is perfectly OK. It even helps spreading the costs for those who have low income as Independants.
Personally, I like the subscription model of Agenda. It gives for free basic functionalities and more for the paying Pro. When you decide not to renew the pro features, you keep all those you already have and are ensured to still have updates. Just that you don’t get new Pro features.