True, but, it would already be better than we have now where we default to 1 rune.
But, it is as you say, the idea is contingent on mechanical distinctions between runes.
I’m going to refer to V Rising, again, since it just does it fantastically.
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Boss that summons mobs → AOE Ability
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Boss that shoots projectiles → Shield Reflection Ability
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Boss with a hard hitting ability → Counter Ability
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Boss with a dash → Gap close weapon ability + Ranged Ability
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Boss has an extended punishment window → Enrage Ability that enhances Attack Speed.
In essence you swap abilities for different instances and combat is so fluid, I highly recommend you watch a V Rising Brutal Boss video. It looks and plays so 
ing good.
Imagine if Wicked allowed for this, where we have a shield rune that can reflect the echo knights projectiles back at him. He uses his eruption/charge ability we use a leap rune to jump away. Then when we have staggered him we use an ultimate rune/enrage rune.
I’m with you 100% on mechanical distinctions I just want them paired with cooldowns and more customization.