Why have rune "levels"?

I'm referring to the fact that we cannot select higher-grade runes without getting the others first. This is a problem because it limits their viability on some champions; for example, Conqueror could be borderline broken on Mordekaiser if he didn't have to go through the whole of the Precision tree first, whose runes and bonuses are not that great on him.

Imho a solution would be to make the "levels" separate groups/trees.

For example (just to show how it would look like, please don't fixate yourselves on the single runes/trees - I'm making them up on the spot):

#(Can only choose 1 rune per group!) Laning tree

(Here I'm using "creep" as a synonym for both minions and monsters, pls don't kill me)

• Additional on-hit damage vs creeps
• Additional aa range vs creeps
• Killing creeps restores mana
• Killing creeps restores hp
• Single-target spells apply reduced damage but as an aoe (only against creeps)
• Killing creeps slightly heals allied creeps in a small area
• Creeps that die near you without you killing them give you some gold
• Creeps that die near you without you killing them give you some hp
• Creeps that die near you without you killing them give you a temporary damage buff vs champions

Mobility tree

• Occasional burst of movespeed when attacking champions
• Additional out of combat movement speed
• You are permanently slowed but can walk through terrain - sorry, I tried to avoid using disadvantages to make up runes here, but I'm not that creative :(
• You cannot be displaced but all of your damage applies as a delayed damage-over-time effect and CCing enemies temporarily slows you down
• You cannot be slowed but enemy attacks can blind you (you know, the visual effect when Graves uses his smoke grenade)

Offense tree

• Your first damaging attack or ability after you enter combat Grounds its target
• Your first damaging attack or ability after you enter combat temporarily reduces its target's damage output
• Your first damaging attack or ability after you enter combat marks its target; after a few seconds, the mark explodes dealing bonus damage (based on the target's missing hp)
• (Insert most of the current keystones here, they're almost all pure damage increases anyway)

Scaling tree

• You can buy "gold potions" that increase your active (from farming/killing/using support items) gold gain; when they expire, they refund their cost (only partial refund if you died during the duration).
• Your trinket gets reduced cd the more you use it
• The current Dark Harvest
• The current Grasp of the Undying
• Stacking %cost for abilities when you hit enemy champions

Ok, can't think of anything else, these will have to do as an example.