Percentage armor reduction and armor penetration stacks multiplicatively. Depending on whether or not it is total armor penetration or bonus penetration all these armor penetration apply accordingly to base armor and total armor. Also note the difference between armor penetration and armor reduction. Armor reduction actually reduces armor so it increases physical damage dealt by everyone on his/her team while armor penetration "artificially reduces" armor for the wielder of armor penetration so only the wielder deals increased physical damage.
If someone builds these items
, and is able to stack up armor reduction from the Black Cleaver to 6 times on an enemy champion (which is tough on most adcs because they have to focus whoever is the closest, unless you are
, etc...). That enemy champion will have both their base armor and bonus armor reduced by 30%.
Total armor= 0.7x + 0.7y, where x is base armor and y is bonus armor
7% armor penetration comes into play, but this time only the wielder benefits from it:
Total (artificial) armor = O.7(0.93)x + 0.7(0.93)y
45% bonus armor penetration from whatever Last Whisper upgrade we have:
Total (artificial) armor = 0.7(0.93)x + 0.7(0.93)(0.55)y
And then flat armor penetration comes into play:
Total (artificial) armor = 0.7(0.93)x + 0.7(0.93)(0.55)y - (20+10+10)
If we use your example, someone with 100 base armor and 200 bonus armor, his/her resultant armor should be
0.7(0.93)(100) + 0.7(0.93)(0.55)(200) - 40 = 96.71
Stacking multiple percentage armor penetration and percentage armor reduction items/abilities/passives together would only result in diminished value compared to the hypothetical situation where they stack up additively. Sure it still reduces a crap ton of armor, both literally and artificially, but the wielder has to damage the same target 6 times. Having 200 bonus armor late game means that this enemy under attack is most likely a bruiser (ignoring the fact only some of the true tanks have 100-ish base armor in late game and they will most likely build more armor than that anyways and have some sort armor/defensive steroid even if they build only 200 bonus armor). If someone with these items/runes/masteries is allowed to burst bruisers without getting retaliated by gap closers/cc/peeling/escapes from the enemy team to prevent them from stacking the Black Cleaver, it's utterly unrealistic unless you are talking about extremely low elo players and new players. For true tanks to get armor shredded by the Black Cleaver 6 times (with also more armor than that), the wielder of these items would most likely be eaten alive by the rest of the team, given how the tanks are most likely on top of him/her. On top of that, the less bonus armor an enemy has, the more diminished the value of this build is. You only most often see this build when the wielder is facing a very tanky enemy team (
), you won't see someone build both
/
and
against tankish bruisers because it's not very effective against them.
As for your magic reduction/penetration formula (the relationship between magic reduction and magic penetration works just like that between armor reduction and armor penetration), I will just do a quick calculation (counting runes in a realistic view and masteries and assuming the wielder of
is level 18 and takes Cunning tree in their mastery page and unrealistically someone has 100 base magic resistance... <on a quick note, the highest base magic resistance a champion can have is 53.4, which is way lower than the highest base armor obtainable>):
Total resultant (artificial) magic resistance = (100 - 8.333)(0.65) + (200 - 16.667)(0.65) - 7.83 - 8.4 - 30 = 132.52
If we have
as well and it's also fully stacked on a target,
Total resultant (artificial) magic resistance = (100 - 16.667)(0.65) + (200 - 33.333)(0.65) - 7.83 - 8.4 - 30 = 116.27
If the wielder takes Ferocity tree instead (
),
Total resultant (artificial) magic resistance = (100 - 8.333)(0.93)(0.65) + (200 - 16.667)(0.93)(0.65) - 7.83 - 30 = 128.4075
If the wielder have
fully stacked on a target and take ferocity tree,
Total resultant (artificial) magic resistance = (100 - 16.667)(0.93)(0.65) + (200 - 33.333)(0.93)(0.65) - 7.83 - 30 = 113.295
Honestly even if we don't include Wit's End, the numbers of magic resistance and armor are not that far off from each other, not to mention how most mages have more burst damage than AD champions (some are more so than others like
) and CC to allow them to deal even more magic damage and do more spell rotations. Many mages are more CC-packed than ADCs and some bruisers as well. It's also not plausible for one to stack 200 bonus magic resistance unless they build 4 or more magic resistance items (which makes the only ones capable of doing so tanks) and it is also practically impossible for one to have 100 base magic resistance. Also having more magic resistance means percentage magic penetration will be more effective on them, compared to bonus armor penetration from
, as it penetrates both base magic resistance and bonus magic resistance. Mages also stack more AP than AD champions stacking AD. Most FOTM mages either have high AP ratios (for burst damage) or consistent magic damage packed with more CC to allow them to actually deal more damage in a spell rotation than an ADC/bruiser will. If they aren't focused down or they are pseudo-tanky mages (
, etc...), they are more destructive than an ADC with their (AOE) CC and damage once they get into range, hence there are more armor reduction/penetration options than magic reduction/penetration options.
There are always exceptions to the above statements, but all in all I think armor penetration is in a good spot right now.
##TL;DR Please understand how armor/magic reduction/penetration work before commenting on the issue.
#(Realistically speaking, magic resistance should be treated this way,
#For mages who take Cunning tree,
#Total resultant (artificial) magic resistance = (53.4 <highest base magic resistance obtainable> - 5.268)(0.65) + (200 - 19.732)(0.65) - 7.83 - 8.4 - 15 or 30 = 117.23 or 102.23
#For mages who take Ferocity tree,
#Total resultant (artifical) magic resistance = (53.4 - 5.268)(0.93)(0.65) + (200 - 19.732)(0.93)(0.65) - 7.83 - 15 or 30 <most likely 30 because most mages using Ferocity tree make use of
as well> = 115.2378 or 100.2378
#The numbers are not as far off as you think they are compared to armor...)