KEEP URF MODE. (Reasons and arguments inside)!
Riot, you have made arguments and reasons why you can't keep URF forever.
Here are some counter-arguments (source is in the link):
Dismissing URF’s long-term (or at least longer-than-two-weeks-term) viability by saying it’s a) too expensive to maintain and b) not maintaining enough interest to warrant the cost doesn’t hold water once you consider the fact that Riot maintains a few incredibly obscure game modes the vast majority of the community doesn’t play and has no interest in trying out. Exhibit A: the rapidly dwindling “Dominion”—a game mode that, not unlike URF, actually began as an experiment. Riot could easily switch Dominion and URF’s places, and would probably make a lot of people happy in so doing.
Even if the developers felt like they were “treading water balance-wise,” did they really need to? URF was meant to be ridiculous and broken. That’s what many people loved about it—at least for a short time.
Seeing as League of Legends’ “normal” game modes have been changed on a monthly (if not weekly) basis for six years now and Riot describes the game as “always-evolving,” two weeks is nowhere near enough time to evaluate the success of URF. The “wildly toxic champion play patterns” that began to spring up may just have been growing pains the community could have worked through. People love to pigeonhole diehard, vocal fans, but give the League hivemind some credit; they do end up solving a lot of problems and establish positive community norms by talking through them together ad infinatum.