I'm perfectly ok paying for an upgrade ( or even full price if it's not expensive ) for new versions. I have some pro audio and video software that costs over 200$ for upgrades to new versions. I just don't like being hold hostage by software that has no jusfication for monthly rent.
Some subscriptions are just money grabs like the adobe apps, but many other apps like Evernote and day one actually need a stead source of income for years and years of increasing cloud storage and server costs. Not defending airmail here since they seem to be more in the former category but there are apps that truly need the subscription model. Personally for those apps I prefer apps that I can actively support when I want my data to be stored on their servers for a long period of time since it will ensure the company will stay around for longer.