People buying less in response to higher prices isn’t a decease in demand. It’s a shift along the demand curve, like so
https://qph.cf2.quoracdn.net/main-qimg-4c7fea30b91c1304b61ef22f90981e39
You’re confusing quantity demanded, which is the amount of stuff people will buy at the current market price, with demand, which is a function describing the amount of stuff people will buy at every price.
Demand has only decreased if people become less willing to purchase a good at the same price.
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