But this has not happened. You're basing a decision today on something that might or might not happen in the future.
"One day," Apple could do anything. One day, they could totally prevent installing things that are not notarized. One day, they could entirely prevent non-AppStore, non-sandboxed applications from running on macOS. They haven't, but they could and indeed all signs point to these being on the very long-term roadmap. Does this mean you stop writing Mac apps today?
In fairness it happened then unhappened due to a mistake. I think people are more worried about the sort of semi-arbitrary decision making associated with the App Store. But, to their credit, the macOS team have never used Gatekeeper that way. It's been >10 years that I've been reading predictions that they will, but they never did. That must count for something.
"One day," Apple could do anything. One day, they could totally prevent installing things that are not notarized. One day, they could entirely prevent non-AppStore, non-sandboxed applications from running on macOS. They haven't, but they could and indeed all signs point to these being on the very long-term roadmap. Does this mean you stop writing Mac apps today?