More and more gamers are using macs, and Apple products make up majority of all games purchased recently. Sure most of those are mobile, but regardless the apple world is expanding rapidly and there is nothing wrong with requesting a native client.

Put it on the app store or release online through account downloads? Sure why not. Most files and content get to be reused. If you've played a game on mac that's also for PC, you see most of the addons work, as do save files, from windows. Like you can install Neverwinter Nights expansion packs for the PC on the mac and they work just fine. So it's just rewriting the main executable to be on mac and keeping all the other content the same.

Some companies will do it for you, like Aspyr. Just pay them to do it and release the updates from square enix.