The same argument could also support a position that armor is "toxic" to League of Legends, especially in a hypothetical world where armor pen was removed. At 100 armor, you have 50% damage reduction, which cuts the value of the offensive stats a marksman or physical damage caster is buying by 50%. It's a two-way mechanic, where each group has a way to "negate" the other's purchases.
You didn't specify if you were referring to marksmen or AD casters or both building these items. Trying to determine if it's the optimal build for AD casters is more work than I want to do, as it will depend on ratios and bases. For marksmen who are primarily autoattacking, this is not the optimal full build in your scenario. Trying to determine which has better powerspikes is also beyond the amount of effort I want to put in, so I'll stick to full build analysis.
The rest of this post is to show my numbers and reasoning for my marksman claim
Assuming a 100 AD without items marksman that took the 7% armor pen mastery and Fervor of battle vs the aforementioned 100 base, 300 bonus armor,
The "full penetration" build
(assuming a fully stacked cleaver, which is not fast for many marksmen)
does 117 per auto with Fervor of Battle doing 5.6 per stack up to 162 damage per auto with fully stacked Fervor
The more standard marksman build
(notice this is not a full build either)
does 148 per auto on average (more or less depending on crit luck) with Fervor of Battle doing 4 per stack up to 180 per auto with fully stacked Fervor, for 26% to 11% more damage per auto.
The "standard" build has
- 40% attack speed all the time instead of only during the Ghostblade active
- lifesteal that can give effective health in the form of a shield
- can apply a 25% base + full Fervor to up to 2 other targets
- another item slot left to put anything else
The penetration build has
- a damage proc
- more movespeed
- more health
- MR and a magic shield defense
I picked Mortal Reminder over LDR both to have one less thing to worry about calculating. If I had included it, it either doesn't change the outcome because the target has enough health or swings slightly in the favor of the standard build because of it's lower actual health.