You mean, assuming there was only one Bitcoin: if a VW Beetle costs 1 BTC this year, and next year a Porsche is published (growing the economy by replacing cars with better cars), the Porsche would also have to cost 1 BTC because there isn't more than 1 BTC?

That seems incorrect. While impractical, there might be other solutions, for example the Porsche could cost 1 BTC+ 1 Carrot, or maybe it could cost 2 BTC anyway. The buyer just wouldn't be able to pay in one go.