Gonna chime in here and also say I liked the light alphas.
I don't think it's necessarily valid to draw the conclusion that the dark alphas are overall more popular than the light alphas based on sales of the last time this opened.
If I recall correctly, the dark alpha set was cheaper and included fewer keys. Mainly, it got rid of a lot of the cruft that the main base kit with the light alphas had.
I know I personally did not join the GB for the light alphas because I didn't want to pay so much for a bunch of keys I would never use. I wonder if others were in the same situation.
I would love if two changes happened:
1. The Tab key in the extras moved to the base kit
2. A new "light alphas" kit came back, with spacebar and numpad support
With these changes, I could effectively build 2 keyboards by buying the base and alphas kit only.
In the way you had it setup before, you forced buyers to choose between dark and light alphas, which definitely cannibalized sales as you stated in the original post. With this approach, everyone has to buy a base kit. If someone just wanted light alphas in the base kit, they would have to spend more to get the base + alphas kit. But alpha kit purchasers have the ability to build 2 full keyboards, which makes the added cost more palatable.