URF has always been very popular when it has been online. I heard the reason Riot doesn't want to keep URF available is because they noticed it causes a decrease in overal players whenever they closed URF, while probably not creating many new players for having it online. To have URF available permanently will affect the ranked/normal Q times aswel.
That being said, I think they should stop being so dictatorial and overly protective and just make all custom game modes available, that would give enough options for a whole new playerbase to join the game: casuals. Casuals are an even better demographic for selling skins than the competitive playerbase that LoL has right now.
All I care about personally is Ranked and URF, and for them to take it away everytime is just getting quite frustrating after a while. It sends a bad message, like they don't care what you personally, as an individual, would like to play.
Right now they force us to play what they want us to play, which is not how it should be done in the long run if you want to serve your customers.