Well, the pirate potion costs 150 ancient coins. And it seems that once you choose your new character you can never go back to the original one.
I wish we could have more than one without losing the others.
As I said, they could make it that each pirate has their own inventory and unlocked cosmetics (so if you want an specific outfit for multiple characters you would have to play as each one of them and unlock it on each pirate).
Or they could make it that Pirate A is free, after that you could pay like 500 ancient coins or whatever the developers would choose to unlock a slot for Pirate B, and a other for 500 for Pirate C and so on, but that means they could share the same inventory from your account. Pirate A, B and C can use any item the others unlocked.
In my opinion any of these methods would not invalidate the appereance potion. (Also they could make it that you pay ancient coins for each extra slot and on top of that each pirate has their own separate inventory).
Some people would like to play as more than one pirate. Unfortunately for us the game only allow us to have one at the moment. I don't see any negative impact on having multiple characters.
I personally don't understand the argument of "Your pirate is you, you can't inmerse yourself if you are constantly switching". I think inmersion depends on the player rather than the game, some may take it seriously other don't, I'm a very skinny guy in the real world, but I wanted to play as a fat pirate in the game, it doesn't reflect who really am I. I just want to play as a female pirate every now and then without losing the fat guy I've been playing since launch.