Oh yes, the old "there's no such thing as a right triangle in real life, only in mathematics" argument.
I'm afraid I don't have a good answer for that, but I'm curious, how ever do you manage to use a tape measure? /s
Your demand curve has less to do with price than it does with whether or not you've seen a billboard for ice cream or it happens to be a hot day and your kids are in the back seat screaming for ice cream.
Economists call that a change in demand.
Also consider that cost includes not just the $5, but the drive to the store, tolerating screaming kids, etc. $5+(drive to store) is much more expensive than $8+(right next to you while inside a movie theater).
Most of the time, it's accurate enough to combine these costs into the price and call it a day.
Why do you think they're able to sell so much premium ice cream at $6/cone?
This is totally possible if some people have a demand curve above $6/cone. That doesn't even sound unreasonable, my last purchase of an ice cream cone from a retail storefront was 8USD after tax.
People go to Starbucks when they can get a coffee at McDonalds.
The nature of a good is well-defined in economics. If there's any reason to distinguish between two instances of a good, then they're not the same good. Any other conclusion is a violation of ceteris paribus.
For simplicity's sake, unless there's a need to talk about competition, substitute goods, etc, we talk about one kind of coffee, one kind of ice cream. Same thing as neglecting the gravitational pull of the sun in physics.
Is it because there is a smaller supply of Starbucks coffee?
Dunno. Different sellers will have different supply curves.
The argument is based on a notion of cardinal and/or ordinal utility of commodities, but neither the cardinal nor ordinal utility can be measured (or even observed).
Yes, utility is ordinal. But we're not measuring utility, we're measuring price, which is objective exchange ratio: I give up $6, you give me an ice cream cone, the price is $6/cone. (A cost is also a ratio, but the usage is slightly different.)
The circularity of the argument can be described as "Utility is the quality in commodities that makes individuals want to buy them, and the fact that individuals want to buy commodities shows that they have utility".
Utility has to do with the satisfaction of a person's inherent, subjective wants (including needs), irrespective of goods/services. Sitting on my couch right now, not trading with anyone, has utility. Yet in a little bit, going to the store hopefully before it closes will have more utility.
You don't need utility to apply the law of supply and demand, however.