I used to play starcraft 2 before this game so I can't really compare to what this game was, but I can definitely tell that it seems to be making the same mistakes starcraft has.
It seems to be focusing more on what is fun to spectate than what is fun to play. Yes watching zoe instadelete someone is fun, watching yasuo turn around a 1v1 is fun... playing against any of that? Not so much.
How does this apply to game length? People don't like watching long laning phases and long teamfight phases because they don't feel the tension of it all as much as the players. People would rather watch their favorite team roflstomp their opponents in 20 minutes.
Players on the other hand, clearly like the chance to scale and the time to come back from any early game mistakes they/their team may have made. Laning against someone that has 2 kills up on you now means they can dive 1 shot you and take 1 turret shot if you're lucky. Can you imagine how fun it would be if you could actually comeback from that via outplays most of the time? If killing someone 2 kills down on you wasn't completely effortless as it is now and took similar effort as the first 2 times, then both of the players would get a chance to play the game instead of either getting one-shot or one-shotting.
Then the lategame has a chance of being more even because everyone had time to show their skill - if one team is truly better than the other, they will still be really far ahead, but this won't be off of just 1 or 2 mistakes by the enemy, but by a full length laning phase.
so 35-40 is a really good number for games that last long enough but not too long. Although personally I would like at least half of my games to be 40-45, because I enjoy the sense of bonding with the team in long games.
The transition from this attitude: "gotta flash so I can KS and carry my feeding team"
To this attitude: "gotta flash into that projectile (instead of getting kill my team will secure without me) to save my adc even though it will kill me and I'll have to use flash"
is my favorite feeling in league.