Unique Passives - Maybe diminutive instead?

Sometimes itemization can be hard if a variety of situations present itself. Sometimes buying a combination of items feels like wasting x amount of gold.

Some examples of this might be the Cold Steel Unique Passive that reduces atk speed when hit. Kind of pointless against Jhin if I buy Randuins to reduce his crit damage more. Yes it might be useful for other enemies on the team, but if I was going to get a Thornmail too, that is basically 1k gold wasted.

Another example might be Catalyst of Eons. Personally I used to buy Rod of Ages and Abyssal Sceptar (rip) on Swain. Now it feels like I need to pick between them and the GPL hextech item or waste 1100 gold.

We have passives that do stack: Energized, heals/shields strength, and I think Dark Seal still does to an extent (as far as potion healing)

Now, I'm not suggesting completely removing all the limits. Teemo knows that will lead to horrible things. I am suggesting diminutive returns as opposed to none. In terms of Cold Steel, it reduces atk spd by 15% for 1 sec. If I bought both I could see reducing atk spd by 30% being too powerful, especially if I have a Frozen Heart with an aura too, making it 45%. But what if I only got like half the benefit from the second cold steel? So 22.5% pre Frozen heart, 37.5% after Frozen Heart. That negates just over 1 atk spd item of an adc? But honestly, am I really going to build all three unless and all ad team?

Otherwise, I would suggest different build paths/finished products. Like a Thornmail with the Cold Steel effect or a different path to build a Thornmail with a QSS and mr but less armour.

To be clear I am not advocating for all Unique Passives to be changed; some are good, like the gold gen items (The thought of a support having all 3 at the same time actually scares me).