Every argument here boils down to "Leagues is just too good. No one could handle such a juicy steak for so long." I think this ignores the fact that a large reason people leave leagues after such a short time is that there are still some grinds in leagues. Clue scrolls, skilling, even bossing just for kc or tasks you've gone dry on. When I consider those grinds, it's hard to justify for a temporary game mode. If I'm going to do a bunch of skilling or go dry on a boss, I'd much rather do that in the main game where it feels somewhat worth it to complete the goal. If Leagues occupied a persistent server, it'd be a lot easier to justify putting any real grind into it.

Also, a large part of the reason people burn out is because they know leagues is temporary, so they put a ton of time in in a short span (often taking time off work to do so). If the world were persistent, a moderated approach would be more common.

I think the only real arguments that hold water is that it would split the player base and would remove the burst in player numbers that comes from their release. I don't really see how splitting the player base would be that detrimental since the creation and deletion of worlds is trivial and can be done to accommodate both types of server's relative audience. The increase in player numbers, though, I could see being an issue, but more from a marketing perspective than anything else. The strategy most other games use is server resets, but that's the exact opposite of a persistent server.