What Riot Should do with Zhonya's Hourglass

Seeker's Armguard has a stacking mechanic which just disappears into the void when it's upgraded into Zhonya's. Why not keep these stacks to allow Zhonya's to be a strong late game item which you have to invest time and farm into rather than just gold?

Something along the lines of

Armour: 45>30 AP: 100(70)>60 CDR: 0(10%)>10% Active CD: 90(120)>90 Cost: 3500(3000)>3200

New: Unique Passive: Killing a unit grants 1 stack, up to a maximum of 30 stacks, stacks transfer when upgraded from Seeker's Armguard. A stack grants +0.5 bonus armour and +1 bonus ability power, up to a maximum of +15 bonus armour and +30 bonus ability power at 30 stacks.

This way Zhonya's is a pretty efficient item when it's stacked (98.7% with the cost I listed) but when it's unstacked you pay heavily for the active (68.9% efficiency at 0 stacks) and it's a lot weaker until you farm it up.