Not sure what you want to achieve with this thread. A discussion of the bias?
I can do that a bit if that is what you want to delve into. Please correct me on any mistakes most of this is not well grounded in my experience.
It is my assumption that mid lane players in general heavily prefer assassins to mages. I think there is a small number of people who really like specific mages but overall assassins in general and the assassin VS assassin matchup in specific is much more enjoyable for mid lane players.
I think that for mid lane players, assassins provide (a) interactive laning experience, (b) mutual kill pressure (=fun), (c) easy access to macro play, fun in mobility, (d) well defined high points in killing the other team's squishies and (e) higher levels of agency (the ability to make decisions for own and other team) than mages.
In contrast, I believe most of the disadvantages of mid lane playing an assassin are shouldered by other lanes.
When mid is an assassin, there are less opportunities for ganking for other play making classes (junglers and supports) because outright ganking and killing the hyper mobile mid is less relevant. Therefore playing around mid is more about siege and pressure, which is a less fun play style.
Additionally, because of the high reliability of assassin kits, reactive kits on supports are favored to proactive kits (lulu, janna > leona, alistar), leading directly to a loss of the agency of the support player. likewise the ADC players has to be much more careful when entering fights, and also experiences a corresponding loss of agency because of that.
My bias against assassins come from my desire to play a proactive play-making role as a support. overbearing mobility and kill pressure from the enemy mid lane and overbearing value to being able to reliably protect my own ADC, as opposed to other qualities, goes directly against the line of play that i find enjoyable.
I think there is a measure of adaptation that is expected and reasonable that i make when i go into the game. but if a set of very common picks make me constantly having to adapt away from the play styles that are fun to me and into play styles that are less fun, then there is a game balance issue, at least IMHO. And ofc this cuts both ways.
Ideas ?