Strong card flow, no one cares about life loss in Commander, and you can add loyalty counters to your other 'walkers without pulling planeswalkers and then you can add in +1/+1 counters to your stuff and counters onto Helix Pinnacle to win and then poison on foes and then also -1/-1 on their stuff
Theres also Thrull Retainer, which is the cheapest (and most reliable) version of this card in mono-black
(Until your next turn, that creature attacks each combat if able and attacks a player other than you if able.) Legendary Creature - Advisor Dwarf Gwenna, Eyes of Gaea : Add two mana in any combination of colors
If this is the only way to kill off creatures in my entire deck, I might value it highly
Simply put, when a card becomes this valuable, changes in the metagame simply dont cause the price to drop all that fast
I dubbed the issue the Oat Bran Problem