The problem usually lies in the fact that this is the only game people play, or its one of the few that they play. When you play games especially one as repetitive as this its going to get boring and stale. The game has always been like that. Its just when you are new there is so much to be explored when you start out a new game. All the champions are differents, the different mechanics and roles you can play, learning all that makes you think you are having fun. And now that you have learned all you can (in terms of champions/roles) you start to see how repetitious it is, especially when you see the same champions in the same roles, in the same mode.
Its one of the reasons why people find modes like URF so enjoyable. Because you arent taking the game seriously and are open to play champions/roles you arent use to, its something new for you to enjoy, and with that is the reason why Riot doesnt make it a permanent mode. That sense of enjoyment wont last months, years down the line when all you start to see is the same champions in the same mode playing over and over again!
If anything you do (doesnt even have to be gaming) starts to get bored just quit, and find something new to enjoy. You make it seem like League will be forever lost once you leave and come back months/years later. its not. you can take a break. I took one during my first year of playing leagues because i knew if i played continuously i would get burned out and i didnt want that to happen with this game because i knew i would enjoy. I have been enjoying this game ever since. There are times when i go on weeks/months of not playing and im ok with that. Because when i do come back there is always something new, whether its a new champion, set of skins, new game mode, or what ever it is.
All in all if you have to take a break, do so! League will be here, we will all be here. Go do something else you enjoy, or find something new to enjoy. If you arent having fun anymore why continue to abuse yourself into thinking this is the only game you need to play?