But that's the whole point. Just because everything else is always just predictable, a bit of randomness is needed.
I don't agree. The unpredictable part is what players do. That's more than enough for the game to work on. It works for mages, supports, bruisers, tanks, so why wouldn't it work for AD carries? Randomness is not necessary to make a game unpredictable. Player interactions are the best source of unpredictability, and they are more than robust enough to make the game work.
The "stale" I refer to is the fact that, because everything else is exactly predictable, if you make crit a "certain" thing the game will eventually get to a point where the outcome of a battle is yet decided on an equal skill level. This will lead to an even more definite meta, that's where will get to a point which is in a stale, where you can't really go beyond those calculated things and picks because no other result is more efficient and thus winning
Two things: first, everything wouldn't be predictable because the entire point is having player decisions be the main factor in determining success. They always have been. They always will be. This will not change when crit goes.
Second, do you actually think crit prevents this in any way? I don't. I don't play weaker champions because "oh, this dude does less damage, but maybe I'll luck into more damage than the other guy even though it's very statistically unlikely". It doesn't work that way. No one makes that choice. Randomness is not the magic mechanic keeping anyone viable right now.
As for stale, here's my definition: If I reach a point where I don't have to think, then it's stale. Once I have crit, I have no more decisions to make about it. The randomness doesn't affect my gameplay. Because of that, it's stale. I'll accept that you define stale differently, but I don't think crit works for your definition either. It's not preserving any playstyles.
you would see how crit becomes a mechanic you can build your play around just like the other ones, but with the benefit of add that element of randomness needed
I disagree with this statement very hard, and this where I think your entire Runaan's argument falls apart. Say that I could build Runaan's and do just as well as with crit. That would be fine. However, that doesn't change the fact that crit itself wouldn't change a goddamn thing about my playstyle. I may make the choice to buy crit, but then I make absolutely no other choices once I actually have it because the best option is "pretend there is no RNG involved". I can make choices with hurricane about who to hit to get the best bounces, or whether split pushing with it might be a better use of my time. With
Crit is a boring mechanic once you have it. It's completely dull. I think the entire mechanic is terribly designed. That's why I think we could replace it with a %damage steroid without losing anything.
In LoL this same random element can be controlled much better because it come from itemizations/champ kits/runes
This isn't controlling it, it just determines whether or not it is present. Controlling implies that some choice can be made to either optimize or minimize the effect of it. I can't make any choices that optimize crit outside of just treating it as a dps steroid and mathematically maxing my dps. Whether or not I receive gain from unusual activations is completely out of my control. That's why it's such a terrible mechanic. Once you have crit, there is no thinking about it anymore. The best option is to ignore it.
Final point: you give randomness way, way, way too much credit for keeping different things viable. I think you're just completely wrong here. I think that the randomness crit gives doesn't keep any champions in play that wouldn't be in play anyways (Try might be a small exception, but he's the only once). I don't agree with you on this point. It's just not true, and nothing you've said has persuaded me on it. I'm not sure there is anything more to say at this point.