Spanish airline Vueling will begin accepting bitcoin and other cryptocurrencies as payment in early 2023.

"Ticket prices will still be displayed in euros"

How much will the airline keep in Bitcoin and how much will it immediately dump for euros?

Think about this for a second. 

Someone sells Bitcoin to buy literally anything.

The merchant gets Bitcoin. Unless it keeps Bitcoin, a new buyer is needed.

What is the new buyer willing to pay for Bitcoin?

Preference Shift

Selling Bitcoin to buy an airline ticket, a boat, a coat or anything else implies the buyer of the boat, the coat, or the airline ticket now prefers that item over Bitcoin!

And on top of that, the Bitcoin seller has to pay taxes on their gains. 

Hey, weren't we told it was irrational to sell Bitcoin at these prices? Those same people cheer nonsense of some random merchant accepting bitcoin.

Hooray!?

Cashing out puts downward pressure on Bitcoin. And selling bitcoin to buy anything is cashing out. 

Tradeoff 

There is a tradeoff, so it's not all one sided. The tradeoff is greater acceptance. But that tradeoff comes at a cost.