I don't think that I would want to part with the amount of cash that a lifetime sub would cost unless the game had the potential to keep my interest for many years, kinda like XI did.
I would absolutely welcome 6/mo or yearly subs though, but with legacy I feel like I'm getting a great deal as it is so I wouldn't mind monthly either. I think for convenience sakes that yearly/lifetime subs would be great for the company, especially if the majority of those that got them didn't get real use out of it (such as say students who absolutely must take time off for studies and the like), but even if there were many, many players that did make the most of it, this would be awesome for square since money earned is totally better than money lost, and there will be plenty of people paying month-to-month so they'd still turn a profit at the end of the day. But with a lifetime sub they're guaranteed a nice lump sum instead of someone fizzling out after two months if they didn't really gel with the game or have time to dedicate to it.