The simple truth is they're not the same product. I understand that it's frustrating that the Mac version isn't well optimised but it doesn't change the fact that it's not the same product as the PC or console versions.

When you pay for FFXIV the game you're not paying for the game as a whole, you're paying for a specific version of a client that runs that game.

Your account however is a single product that can access all versions of the game client which is why you do not need to pay separate sub fees to play on multiple platforms.

I understand that different platforms have different requirements and meeting those requirements costs time and money to develop for each platform, but it does suck that you have to pay full price if you want to have multiple clients of the game. It would be nice if you at least got a discount if you already own one version of the game client. And to prevent people from repeatedly reselling cheaper versions of the client SE could only allow the discount if it's bought through your Mog Station account so that each account could access the discount only once per version of the client.