It's simpler than that. People often don't know what they want until they're actually presented with the buying decision. Economists call this stated vs revealed preferences and it's a well documented and understood phenomena.
No, I don't really accept that in the discussion about phones re: size and keyboard.
People do ask the wrong questions.
Yes, I would buy the phone with the keyboard, but the keyboard is not the single most important feature. After the Desire Z there simply was nothing comparable. If it was 400 instead of 300 I would have said no, if it was from a brand I hate because they mess up the default Android install I might have said no. If it was 1:1 like some other phone except the battery being atrocious I might have said no.
The problem is that they are changing 10 variables at a time, including size, cost, software, camera, battery, whatever.
They don't like the black one more. The yellow one caught their attention, but when it comes to the actual buying-decision, they find that drawing attention is not a feature they want.
But the existence of the yellow one helped sell the black one.
