Linking upgrades to their higher and lower tier items.

[chiappelli]

Similar to dota. If I click on improved spirit armor, it would be great if I could directly click to buy just the spirit armor alone if I don't have enough for the full improved version.

 

[pakshin55]

Problem is economy, if bebop could just spam echo shard every game since its now builds from something. He will do exclusively that.
Same for other heroes and builds.
It breaks variety and adaptability. If this to happen you will have only optimal builds in place. And get reported for bad builds.
Also snowball gonna be out of control. One mistake from enemy and your standard fill up buy turned into tier 3-4 core item and now you can 1v3 or 1v6 in certain conditions.