Overbearing defense is the original problem/Defense should be more specific
The resulting "damage creep" is a long line of sloppy bandaids patched over the issue instead of Riot directly addressing the issue:
Tanks and high-durability have it far too good in League. They have massively multiplicative scaling with their durability, between the raw values of straight HP as well as the damage reducing stats of armor and Magic Resistance. There's also the HP regen stat, which is far less relevant today than it was in a past before I started playing.
Also consider that many tanks will often have in-built means of multiplying upon their own defense; a shield, innate sustain, flat or % damage reduction. Given Riot's semi-recent philosophy that everything should scale, this usually means that those defensive steroids will, in addition to directly amplifying their defense stats to begin with, will also increase in their own value off of those defensive stats, e.g scaling with their health or resistances. What this means is that in addition to having a very large HP pool, tanks are also reducing damage based on their resistance stats, as well as whatever metrics they have innately to keep themselves alive. Many of these defenses are completely indifferent to what is hitting them; they're all-encompassing in most regards with few exceptions. They're almost entirely generic stat-buffs, which means that the only way to bypass them is to simply have superior numbers.
Obviously, this looks very problematic. Tanks and very tanky fighters (and tank Ekko) have been issues in the past because of this dynamic, and repeatedly. What this has generally required is that it's not enough to just "do good damage." To deal with all of that combined defense, you have to be able to ignore it. And that's where the bandaids start rolling in.
First it was the influx of % damage. It's so common now that it's basically added as an afterthought to "make damage equal no matter who you're hitting" and nobody bats an eye at it anymore.
Then it was penetration. To compete with the growing power-creep of tanks, Black Cleaver shifted from flat to percentage shred so that fighters could better keep up - as well as the shift from Last Whisper to be % bonus armor penetration, later reverted. Lethality is not quite as relevant on its own but it's certainly significant when one considers that most Lethality buyers are stacking every type of penetration available. Magic penetration is, as it's always been really, broken, but it hasn't changed much other than a few numbers.
Now it's true damage. Fiora, Camille, Conqueror, IE. I don't think I need to say much here that hasn't already been said. Bypassing defensive stats entirely has become an "acceptable" practice when it was normally used sparingly in deliberate ways like Silver Bolts, Noxian Guillotine, or Feast - single target tools that are meant to be target agnostic and not laughing at some poor sap who just wanted to play Nautilus in soloQ.
We seem to spend a lot of time looking at the effects and never give consideration to the cause, which is that tanks are quite dumb. For comparison's sake because I don't play any other MOBAs, I'd like to mention DOTA's handling of durability in regards to tanks and tank-likes. First, one should take in mind that DOTA has 3 "primary" stats - Strength is the one that's relevant to us. Buying Strength gives you health, health regeneration, and a very small amount of MR. If you're a primary Strength hero, you gain more of these and also gain AD for every point of the stat you have. Notice that there's no mention of resistances; most tanks in DOTA are all health, while armor is given to carries (Agility) at the tradeoff of having very poor HP pools.
DOTA's tanks do not usually have generic defenses if they're put innately into the kit; they're generally focused around defending or countering specific things. An example would be Tidehunter, who has a passive that ignores a flat value of AD from auto attacks and a spell that can further reduce the AD of enemies hit by it. This makes him very strong into champions who rely heavily on their AD but utterly defenseless against champions who deal damage through on-hit applications.
There are two tanks I can think of who resemble this dynamic already. Galio, who has a magic shield and whose damage reduction is half as effective vs physical attacks. Malphite, who is incentivized to invest heavily into armor and whose base kit sets him up to be a strong counter to melee AS-based champions. This is a much healthier dynamic as it gives tanks clear-cut win cases and situations to be valuable in that let them do their job without undercutting entire groups of champions. Currently, due to the very generic nature of tank defense (and I would say most defense since Fighters have now adapted the shields and % reductions now), Riot has had to implement generic solutions to addressing it.
Obviously, nobody is happy with the end result.
I don't know what the for-sure correct choice is, but the solution isn't quite as simple as "remove Conqueror" "nerf armor pen." To me, I think that in order to solve the endless arms race between defense and offense that defense needs to be taken back to the drawing board and stripped down to a much simpler set-up that allows for more clear-cut values, with in-kit defenses shifting from generic shields and % damage reductions to more specialized setups against specific champions.
I lost my train of thought here. Bye.