I fully realize nobody at Jagex can say with any kind of certainty, but I'm curious:
Hypothetically, if RuneScape as a whole (including OSRS) were to die, would Jagex consider releasing or selling a standalone, offline version without any continued updates or support?
If the whole RS franchise was to cease to exist it would be sensible to do that depending on the work that needs to be done to allow it to happen. It is rarely a case of just releasing code. You're going to be playing for many many years to come so I don't think we'll ever need to find out.