My reasoning for upping the cost is because the ranger's animal companion is more limited than other classes, and so the drake is a larger improvement. And the original drake warden took away so little only because it utterly ruined the already weak drake by taking away powers and size increases. In my opinion, a little more trade off was called for.
That said, I will admit, I didn't really weigh the effect it has on stacking archetypes, I was focused on balancing the archetype itself. And when weighing balance, I do try to err on the side of caution, both because it does less harm and because it is too easy go in the opposite direction by accident.
But hey, it's all homebrew, if you want to change it, be my guest. I'm just happy to hear you like it.