I almost want to give you money just to encourage this kind of behavior.
in fact there are (at least) a couple of start-ups focused on subscription (recurring payment) management. The one i use is Trim.
this app is useful and paid for itself by a factor of 10 just in the first month
but here's the thing: a very large number of vendors have made their subscriptions resistant to cancellation by these "subscription managers"--even after Trim contacts them, the response usually something like "only cancel the following the tedious manual steps" which even after you do it, you have to check again next month to be sure your cancellation request was honored
Or not pay for v2 because you still have v1 and it works. Or pay for v2 because it has some new features you'd like.
Ok, there isn't a perfect way to pay for software. Developers usually pick the one that suits them better and it can change along the years. The constant is that they need money to cover their expenses.
Either way, users lose. One-time purchase means probably less software (and less innovation in the space). Subscriptions means more shitty software and user-hostile business models.
Still, as it is, the expected lifetime of a piece of software is usually much longer than the expected lifetime of a SaaS business. So if something seems like a tool I might frequently use, I'd much prefer to buy it than to rent it - there's a better chance the company will abandon it or die than that the bit rot will set.
The trial version is not time-limited, but it is limited in two ways: each drawing can have a maximum of 30 distinct objects, and exported images/PDFs have a small watermark along the bottom edge.
Save/load/import/export are otherwise unrestricted.