The former is highly prone to abuse cases, and the latter is much too resource heavy considering that there are millions of players for every Rioter. The latter would also likely result in a lot people requesting it for minor things that weren't even punishable.
I will go into a bit more detail if you want, but a clear example would be cases where someone is simply doing poorly and their teammates mistaking it for intentional feeding (or making poor decisions). High elo players, whom I'm assuming you mean when you say "experienced and skillful" aren't exempt from misinterpreting situations and behaviors, or for mistaking a tilted player from an intentional feeder.
The automated systems in place already detect intentional feeding with much higher accuracy than other players (even more so with reports), and much more timely then a Rioter receiving 1000 admin spectate requests per second, minute, etc.