Now I understand this may be a little click baity and that what I am talking about below may be a little bit ranty, but I think it needs to be said. That said if its TL DR, skip to the part after "The Core Problem"
The game simply lost any fun factor over the last 2-3 years.
Quite honestly it started with the first mastery rework that brought us keystones. Base stats weren't adjusted properly to make up for the new sources of damage brought into kits for every champion in the game. I will admit thought that near that point was the juggernaut rework which to me was the turning point (wasn't as bad as a lot of people made it out to be and I could still enjoy the game), but keystones were the line of no return being crossed.
Damage and tankiness is something that is beyond balancing anymore. You either have stupid damage or are just so tanky that you can just kill squishies with your high base numbers until the enemy team can afford to put 3 damage dealers on you to kill you in under 3s. Bruisers are dead and have been for a long time. Mages are worthless beyond the early game with a few exceptions and mostly because of the problem that mages are basically AP assassins or hyper carries with how the game is balanced right now.
Lethality is literally the exact thing Riot purposely denied for many years in this game. Which was an early game build that scaled past the mid game to everything except the extreme late game. They purposely avoided putting in too many armor pen items and made sure that their next options were fairly unclear or open ended to make sure that armor pen would only spike you are one point and if you denied that spike it was wasted gold. Now its just build more damage or maybe build your only defensive item.
Now if you want to sit there and go but the game is balanced in pro play or anything like that. I want to remind you that the strategy for LCS is literally farm and avoid enemy ganks, take objs, teamfight, then end the game. Split pushing cannot be done anymore in the LCS thanks to mobility creep and damage creep making it harder for champions who traditionally filled those roles to see any play combined with the addition of recall buffs to baron making it less risky to send the team for a relatively safe OBJ at the center of the map that has dragon to contest with it which quite honestly is another major problem for the game (I remember when 1500 team gold was considered worth it, now its like 1500-2500 team gold for a dragon or over 10k with elder because of the effects). Watching a pro blow up in less than 2s isn't fun to watch. I remember watching Faker sit in disbelief dying to a .1s burst from an Anivia back in Season 7 as Zed. How is that entertaining to watch? Sure there's a few good moments, but everything happens so fast now there's a lot less enjoyment to it.
**The Core Problem**
I don't know about you but my favorite thing about League was the fact that some games you would get rolled early, but you still had the chance to come back. You could go 0/3 in lane and still have a chance. Now 0/3 is GG unless your team is ahead enough to deal with it or they throw. I mean we can just look at the team fights now. Fights that used to sometimes take 10-15s are often resolved is 5-10s now. I could at least keep track of what was going on on the screen back in the day, now its like trying to interpret a 4 year olds hand writing and put all the puzzle pieces together during a team fight. I didn't like the extremely fast kill times in DOTA 2 back around Season 3 of league for the same reason I don't like them now in League. It makes you feel like less of a player and more of a spectator and quite honestly it makes it less fun to kill and more frustrating to die. This is also compounded by the fact that half the time there is almost no decision making in builds and quite often the game is decided by the champions you pick not how well you play making you feel like you are interacting even less with the game.
The sad thing is I am finally seeing decent changes to balance on Riot's end this season. The problem is the damage from previous seasons that were supposed to "refresh" the game are just slowly eating the game alive and quite honestly making it a game that is borderline about to die. I understand that tournament viewership is going up, but the problem is that the team fights and ganks may be fun to watch, but the gameplay itself is very lacking in enjoyment outside of super coordinated play. League of Legends is changing from the game everyone wants to play to the game everyone wants to watch and that trips a really big red flag for me as a gamer.
Now I know that this will probably get a lot more strong hate than actual agreement, as that is always the case with things like this, but as a long time player, regardless of skill this is my opinion and I stand by it as I have seen so many people just quit this game and listened to so many people voice their concerns about playing it.