In all fairness, you are wrong
about your first argument. Both demand and supply curves are empirically observable through auction mechanisms and market reaction tests.
I'd be happy to forward you to research on them if you are interested, but don't try and pose opinion as fact if you aren't in the field.
In some cases I agree that an auction will give you an estimate of what demand or supply would look like at different prices. Northwestern university used an interesting variant of a Dutch auction to sell its sports tickets:
But auctions are often impractical. Auction mechanisms that induce participants to bid their true preferences are complicated, and you're still forced to assume that the auction participants behave rationally and optimally.
In general, the point that I'm trying to communicate is that it's very difficult to disentangle supply and demand: much harder than most people realise.
I think OP was stating that they aren't measurable in the real economy, not that we can't construct situations in/mechanisms by which they can be observed.
I'd be happy to forward you to research on them if you are interested, but don't try and pose opinion as fact if you aren't in the field.