Hm. I find the baseless, alarmist fear-mongering here a little worrying.
You should absolutely be able to separate the game from Steam if you wish. Having that option is always good.
But Steam's maintenance or other server issues will have absolutely zero impact on your FFXIV experience. The game will not be requiring any sort of authentication through Steam, as FFXIV doesn't and won't (as it would be ridiculously complex and serve no purpose) require you to authenticate through the Steam API or network infrastructure for any reason.
Literally the only thing changing is you'll have to have Steam open. It won't even have to be online, you won't need the overlay, etc.
It's okay to not want to do that, and to petition them to change it, but don't wholesale make things up or fearmonger about stuff that would literally never happen because you don't understand the platform. Ask questions before getting outraged, to make sure you're actually getting outraged about something that you care about.
My main concern is that Square Enix is either knowingly lying about this, or is making a change based on something they don't understand. There's the possibility of some third factor I'm unaware of (maybe people are able to take advantage of redeeming it via Steam and then somehow using it to amass lots of accounts or something? Maybe for RMT?) but they're super vague, and no other game is doing this. It is absolutely not Steam's policy for you to have Steam open to run games. The service actually has a lot of games that don't even require Steam installed, let alone open.
