You want to know why URF leads to less people playing? I can tell you.

It is a phenomenon I think I realized in MMOGs before I played LoL.

I noticed how I felt after URF. Similar to how I felt after playing a test server in Ultimate Online that let me automatically level up my character to the max.

There is no longer a pursuit of obtaining greater heights. You've already experienced the pinnacle of what a character is capable of, and you realize it wasn't even worth the time to pursue you would have invested in the first place. The easy access to power kind of reveals how hollow the use of your time spent really was once you reach the mountaintop easily. It ceases to have the same perception of value as when you worked long or hard to achieve it.

This sense of pursuit of power is more pronounced in Dota, and honestly it's part of what makes MOBAs appealing from the beginning in my experience. This sense of advancement and leveling up, the pursuit of power, each game you're trying to achieve greater heights than you have before, or trying some new and better build/tactic that requires a big investment of time and gold to pull off and you're not even sure if it will work yet. I don't think that sense of pursuit of power is as high in LoL as it is in Dota (because Dota's more expensive items means you almost never get to fully experience your dream builds, and even if you do achieve that the game is almost over usually anyway), but it's still there in LoL. Which is why LoL is still effected by URF in a negative way.

URF is instant gratification. Why waste 15 minutes of laning and 15 minutes of mid game working up to big abilities, big items, and the mega plays and action when you can just jump strait into it? Why wait for that one time in 10 long games where you get to pull off the perfect combo and ultimate, when URF gives you the opportunity to spam your abilities and pull off these moves with greater frequency? URF gives easy access to unlocking the full power of a character without the need for even having items or levels. You get to push a character to it's limits and play around with things to an extreme that lets you experience more in a shorter timeframe. Consequently, it reduces your tolerance for the slower paced and more methodical nature of the normal game. I noticed this effect in the game World War 2 Online, when they introduced mechanics that made for quicker battles they also reduced the player's tolerance for continuing to engage in the more time consuming forms of battle that had characterized the game for most of it's life.

The problem is not URF, so much as the problem is that the games themselves don't really justify the amount of time we invest in them to begin with. That's not necessarily a knock against the game's quality, but I think it's a general statement of truth about any MMOG or MOBA that people invest more time in them than they should because they are constantly in pursuit of more achievement and realizing their vision of builds/tactics. The instant gratification modes only reveal to players that, regardless of whether or not it's a good game, that it's really not worth the excessive amount of time they have been devoting to it in pursuit of that achievement. It tends to reveal the games for what they are, massive timesinks where the fun bits are buried in mounds of tedium that people generally wish they could bypass. (unfortunately, that tedium also contains a lot of strategic depth, which is why Heroes of the Storm is a shallow, boring game, because they tried to reduce the MOBA experience down to a constant teamfight brawler with no map strategy, no builds. and no ability to gain no level/gold advantages over your opponent that accrue over the course of a game thanks to smart lane play).

I still like URF. I still want it back. It didn't burn me out from wanting to play. But I think I can understand what dynamics might be at work here, because I've had it happen to me in other games.