Honestly, all I ever play is ARAM and special modes. If URF became a permanent mode, I'd probably abandon ARAM and play URF instead. I enjoyed AR URF, because you don't get the same person instalocking Yi and other like champions every game, and you're forced to play different champions (assuming you own more than a handful). That doesn't make URF or ARAM any less infuriating compared to any other mode though, as you'll always get people who regularly play other modes and have the "eh, it's just ARAM" or "eh, it's just URF" mentality. For ranked and even normal games, you get those people with the "lol bro it's just a game, who cares" mentality. How infuriated you get depends on how well you can tolerate other people, what you expect from others, and how they actually perform, which varies.
I personally would support permanent AR URF, but I would settle for constant active special modes - when one mode ends, another begins. Also, it would be great if the mode didn't disappear for like half a year, and that they would be on a more active rotation.