Nobody expected it to live longer then a few months because it would be stagnant forever until they started doing community stuff and OSRS only updates, which is brilliant in hindsight but still CRAZY and unprecedented
Even with updates noone expected it to last this long, at least not Jagex anyway.
Remember when Mod Nexus went back to RS3 after 6 month? Mod Ash was supposed to go back aswell, and OSRS support was supposed to be cut completely. But Ash stayed on OSRS and ever since that point, the team got far more support (engine guy, hired a ton of people, proper development tools, etc).
6 months was how long Jagex gave it and they got proven wrong and ended up being forced to double down on it.