Bitcoin is $0.02. you started with 100 btc, spent 50 Bitcoin and immediately replaced your 50 btc right away with a cash purchase. You're out one DOLLAR. The US Dollar you spent to replace the 50 Bitcoin.

But you still have 100 btc!

If you presume that IF you HADN'T bought the $1 pizza then you WOULD HAVE spent that US Dollar on MORE bitcoin, then sure - in that world you would have had 150 btc. but if you instead just BOUGHT ZERO PIZZA and did nothing else, you'd still have 100 Bitcoin. Not 150!

Where am I wrong here?