It handles baking dishes up to 11"x16".
Most family size single-baking-dish recipes are sized for a 9"x13" baking dish.
> And it can only cook one thing at a time so wave goodbye to roasting veggies while the meat cooks.
While I suppose for some people a countertop oven might replace a full-size oven, most of the target early-adopter market for expensive kitchen gadgets is going to have a decent size kitchen with a full-size oven as well as any countertop oven; the features this has might reverse the traditional "large oven is primary, countertop is secondary" arrangement in those households, but I don't think most people in the early adopter market is going to be using this as their sole oven (though, I suppose, if you were particularly price-insensitive, especially if you already had a cooktop separate from a large oven, getting two of these for a dual-oven setup would be more space efficient than most dual-oven setups.)
And, for those who find a turkey a less-than-stellar example for whatever reason, a 12-pound turkey is a lot larger than a roughly four-pound roast, which (with various different marinades and preparation techiques) is the base for lots of ~6-8 serving main course recipes.
There are lots of people who cook turkey only on Thanksgiving, but it is far from true that no one cooks turkey except on Thanksgiving.
People who are passionate about cooking tend to do it for more people than just themselves.