If increased Bitcoin adoption results in higher Bitcoin prices then why didn't gold prices plummet when we decided to stop using gold as a currency (i.e. gold adoption fell)?
Essentially because we didn't !! We didn't stop considering gold as a monetary asset. People still CONSIDER gold very valuable, and that is why it IS very valuable. At no point in history, gold has lost its monetary value completely. People still consider it as a store of value, and that is what makes it a store of value.
Something is money if enough people think it is money, and for gold, enough people have always considered it as being money. Not money to buy a loaf of bread with (but gold was rarely used for small transactions). But money as store of value.