While I can certainly understand your disappointment, I don't think there's any real lost opportunity here. A cool class idea is a cool class idea. If someone at WotC comes up with one, they're not going to reject it because "Hey, this would've been a cool ki class, but we didn't create that, so I'll just chuck the idea." What they'll say is, "This would've been a cool ki class, but since we don't have that, I should probably make it [insert other power source here]."
I'm not so sure. Which came first, the idea for the warden or the idea for a separate primal power source? If they hadn't decided to make primal its own power source, is there an impetus to come up with the warden at all?
I know they say they aren't trying to fill the grid, but I think each new power source forces them to at least think about the grid, and I think that can generate good ideas that might not have come about otherwise. I was excited to see what cool ideas ki was going to generate, and I'm somewhat disillusioned to find that the answer was apparently "None."