The problem is that demand is not as simple as "agents will buy goods below their reserve price." Real world experience shows us that there exist luxury and inferior goods.
We can quite simply see that there are (many) agents who could buy any number of walmart jeans, but would never set foot in the store. Another chunk of agents currently buy, say, Levi jeans, but likely wouldn't buy those either, if those exact jeans were a third the price and sold in a discount store.
Similarly there are agents who don't give Levi a second thought, but will line up for jeans of similarly quality, trivially variable style and 10x the price.
The real world demand curves for blue jeans don't look like the simple axiom suggests.
I didn't claim the law of demand was perfect or that substitute goods don't exist. I was merely pointing out that wisty was wrong to claim the law of demand requires that "a huge number of axioms hold (none of which are true)." You need only one axiom which is, generally speaking, reasonably accurate.
Further, your example of jeans is trivially explained by noting that "jeans" is a broad category of goods, not a good itself. I.e., there is one demand curve for Levi jeans, a separate demand curve for hipster jeans, and a third demand curve for genuine levi jeans (not jeans that appear at first glance to be Levi, but sold by a shady discount store).
Your critique of the demand curve for jeans applies much better to various demand aggregates in macroeconomics, not the law of demand for specific goods.
We can quite simply see that there are (many) agents who could buy any number of walmart jeans, but would never set foot in the store. Another chunk of agents currently buy, say, Levi jeans, but likely wouldn't buy those either, if those exact jeans were a third the price and sold in a discount store.
Similarly there are agents who don't give Levi a second thought, but will line up for jeans of similarly quality, trivially variable style and 10x the price.
The real world demand curves for blue jeans don't look like the simple axiom suggests.