It's not naive. Bitcoin is already widespread as a medium of exchange. Just look to the whole trading industry of cryptocurrency. It's the most pair of all the altcoins and that's just another use of bitcoin being a medium of exchange.
You're correct about the volatility and supply limit of bitcoin. But that is also the reason why many of us believes that someday it will be as is, payment medium and a store of value due to scarcity.
When I transact, I always convert BTC to USD. I check how much the transactions cost in terms of real-world money. The question to ask is whether volatility will go away at some point. It's clear to me that the answer is No.
1 million: Too small. Explanation: Volatility will go away when we reach 1B.
1 billion: A little bigger but still small. Explanation: Volatility will go away when we reach 100B.
100 billion: We are here, volatility is still here. Explanation: Volatility will go away when we reach 1T.
1 trillion: Game changer?
10 trillion: Definitely big. Hope: volatility will go away (likely false).
100 trillion: World's money. Hope: volatility will go away (likely false).
Volatility is an inherent property of Bitcoin's supply design. It'll be here forever.