If my local ice cream parlor is bold (or foolish) enough to offer a "single payment lifetime ice cream subscription", and I would have got that, yeah, I would expect to never pay for ice cream there again...
Neither the hypothetical ice cream parlor nor Goodnotes is accused of doing that, though.
I don't know when the OP bought his app, but the pricing page from a year ago doesn't say anything about the lifetime purchase being a subscription at all, much less a subscription that includes every new feature in perpetuity.
