I know Guild Wars had original idea in the era of "lets make new world of warcraft and get rich from subscriptions" to release mmorpg without any subscription, you only buy it once and you can play forever.
It was 2005.
You have your timeline slightly wrong because GW1 (if we take dev time into account) basically launched at the same time as WoW - and WoW wasn't the first subscription MMO. I suppose if this was a major reason, EverQuest would be the one to inspire it.
True, I had forgotten about UO, but for the sake of the argument I don't think it really matters. I don't have numbers ready, I had always seen EQ as the bigger mainstream success, could be wrong. Also was a bit before my time, I didn't pay sub fees for my games until 2004/05ish.
Edit: I didn't actually want to say that EQ invented it, but I had phrased that badly.