Percentage damage reduction

I know there are already a lot of threads about how ridiculously OP percentage dmg reduction on offensive items is, but the more the better right?

so basically, squishy carries can now be tanky af while still doing insane dmg, which is just stupid. If someone built Deaths Dance and Phantom Dancer, they already get 42% dmg reduction ( You would need to heal up the dmg from DD, as it just delays it, but with all the lifesteal adcs build its really pretty easy. even dd gives 15% lifesteal) And if you get a tank on your team to buy knights vow, that goes up to 54% dmg reduction.... with 2 items... this isn't even counting the natural resistances you have (armor, MR). like with this build adcs aren't even glass cannons anymore. They're just cannons. If the adc that already has 54% dmg reduction decides to build tabis, now they have 64% dmg reduction against basic attacks. again, not even counting the resistances they already have.

Now this is a hypothetical situation: So your playing graves jungle, the enemy has a master yi jung. its 30-40 mins in and both of you have 4- 5 items.

You have skirmishers with red smite, PD, DD , Blood thirster and ninja tabis. Your top laner put knights vow on you. you find master yi and start fighting him. he already does 64% reduced dmg to u because of tabis PD DD and knights vow. Then u use red smite on him so for the next 6 seconds hes going to be doing 84% reduced damage to you. 84% REDUCED DAMAGE WHEN THE ONLY DEFENSIVE ITEM YOU HAVE ARE TABIS. So this master yi is barely gonna be damaging you, meanwhile your gonna be blowing him up because you have loads of damage from your items, and you have 35% lifesteal to get back the health that you would be losing from the DD d.o.t.

Am I missing something or is this just straight up OP?