especially when you consider the blatant disparity between low and high mechanic designs on champions and that the newer models consistently outclass older ones on most classes
there are too many items in this game that exist as stat sticks which are utterly insufficient for the bulk of "simple" champions (namely juggernauts and pre-kalista marksmen, but I would also add some older assassins and divers and maybe certain mages.) this setup actually plays to their detriment in the name of balance because champions who's kits largely revolve around their numbers but have limited ways in applying them don't really want more numbers to hit you with. They want items that help them get to where they can apply it. Playing other games will make you acutely aware of how League is lacking here, and I'll use one obvious example (dota) and one less obvious example: World of Warcraft. Playing both of these (WOW more than dota I'll admit) has made me hate League's item system and how boring it is.
Dota2 actually has most of its roster as very mechanically simple champions. Its difficulty extends from its macro play and knowledge of the game at large and application is far more valuable across the roster. Immobile melee champions in this game can compete with absurd stunlocks and range advantages through good usage of their items. Armlet allows for toggling to get health and damage bonus but then decay HP rapidly; it allows for temporary survival and damage and so managing the toggle to maximize its benefits becomes very important. Blink Dagger enables flanking and playmaking. BKB allows you to escape deadly CCs and magic bursts. Insane healing is available through Satanic but it's on a timer active with a cooldown. The result is that melee champions remain simple in their own right but maintain high level viability because they are well supported by the game itself and have to be good to succeed.
In World of Warcraft, I'd like to actually compare 2 different classes; Warriors and Warlocks, and how the Engineering profession that enables PVP is a boon to both of them. Both are actually pretty easy to understand, and very impressive when mastered; while these classes are mechanically not too hard, there are some key weaknesses to both that get greatly covered by Engineering trinkets and items provided you're proficient enough with their usage. Warriors are extremely susceptible to being kited, and Engineering is basically mandatory of you PVP as one, and Warlocks have a hard time keeping people away once they're in melee. Engineering skills provide both with CCs and additional gap closers that enable them to outplay opponents and remain competitive against classes that would otherwise be hard counters.
In League of Legends, Garen can run at you. The items he builds only do 2 things in how he approaches a fight; he runs at you faster, and he does more damage if he reaches you. As a result, he must be overtuned to remain competitive. Miss Fortune banks on her ultimate and has no way of preventing immediate death if she gets caught channeling it. Her ultimate must then be made absurdly powerful provided the enemy team is somehow incapable of stopping it. And so on. I would even say assassins preAkali are victims of the same problem; they aren't really useful enough to did anything but kill people, and Riot has decided to make them annoyingly good at deleting enemies without making them teammates you actually want on your team
Almost any time a beat stick champion becomes a problem you can see that it's because Riot has never properly supported them. Unless specific stat items are obscenely disgusting on their most brainless users (Devourer, Feral flare, preArdent Lethality) they're almost always far more useful on champions who can more reliably apply it, because those champions are just better in general. Its the same thing with ADCs right now; KogMaw will do more damage than KaiSa but KaiSa is not a slave to her teammates' competence to be able to carry. Therefore unless KogMaw becomes ridiculously overpowered KaiSa will always be preferable.
The way you solve this is by giving these champions what they need; usefulness and reliability. I've complained about it many times now whenever Riot throws more numbers onto a weak juggernaut. Sure, you've made them *stronger*, but I don't need Darius to be stronger. I need him to be useful. I didn't ask for Nasus to get 50% CDR on Q on his ultimate; I asked for him to not be so hard to justify playing to my teammates.
I really hope that season 11 comes with Items Reforged or something, because I would bet money that as newer and better designs release that beatstick champions who only have items that make them beatstick you harder will continue to become less worth the lock in champion select. A more robust and engaging item setup would be a breath of fresh air for me.