But what if you don't want to buy it as a present? Or know someone with a birthday in January?

I'd still argue that many people are more likely to buy gadgets in December (gift to yourself).

Birthday is different though, but will not be the case for most decisions.

I'd absolutely agree more people buy gadgets in December, but I don't see how that means it couldn't be artificial scarcity.

Long term gains > Short term gains on a product like this.

Because for Amazon the echo is not the actual product, it's just the platform. Don't think they make that much profit at the current price. But people who have one will more likely stay prime members and buy things at Amazon.