But nothing would happen if they kept the game modes in custom. As stated already, If URF was available to play in customs, then the same people who are now begging Riot to release it again would have been done with it by the time Ascension came out. They probably would have been done with it by the time One for All: Mirror Mode came out. So, there would have been plenty of people who would have been playing those game modes by the time they came out.
And, even if the hype for URF somehow managed to continue to this day, and people would still be playing the mode if it was available in customs, don't forget that the amount of people demanding for URF is just a vocal minority. There would still be more than enough people to play other game modes when they came out.