Of course, probably it'll be ok for a while, but then some App Store API or updated processor requirements will render it unusable like so many others (particularly on iOS).
So now I've got several abandonware apps that I paid for: Things (v1), Quiver, Artboard and now Monodraw. I can't say any of them really had any non-minor updates since I got them certainly not features (yes, I realize Things did a separate V2 app ... but I wasn't happy about that happening so soon after buying V1). I do realize some of this is Apple's fault in the way the store is structured.
Too bad Apple doesn't enact some sort of source-code escrow such that if an app is abandoned, the source gets published, but with owner-copyright retained so no-one can resell -- as part of the T&C of submitting an app. Maybe after 1 year of abandonment for a "big" company and 2 years for indie-dev apps.
*EDIT*: Happy to see Milen's sibling comment about fixing breakage!! I realize you have to make a living -- is there a way to contribute for these new features (or is it a totally new app -- which is fine!).