For people like Catwho who've accomplished a lot in XI and don't need to join a delve/skirmish/Aby/Dyna/etc run every night, then I think you can play both games just fine.
Right now, FFXIV is new and exciting. There is a lot to do, so you'll be spending most of your free time exploring Eorzea. And during this time you'll probably spend very little time as your XI avatar. But it won't be long before you'll have finished every quest, beaten every dungeon, and are waiting for new content to come out in XIV that you'll be bored, yes bored. Never mind the server outages and constant emergency maintenance we're experiencing now, and will for many more weeks-months to come I expect. But there you are, bored, what to do? Why jump back into XI of course. Get caught up on the latest content SE has added. Grab the latest gear upgrades, catch up with old friends, get busy exploring Vana'diel again. The more you play XI, the less you'll love XIV.
Then you'll either get all caught up with XI or SE will have come out with new content for XIV and off you go to the other game again! You'll flip-flop back and forth every few months. And you'll be happy. I know, it's what I did during XIV 1.0 through 2.0 beta. Having another game to fall back on when your (current) main game is unavailable is very beneficial.
Yes you don't have time to play both full time, I agree, but not everyone needs to play both full time. Some of us ol' timers are happy with our progress in XI, but not ready to quit yet. There are still a few things on the To Do list we need to cross off first, but are in no rush to do so. So like Catwho, and others, you'll see us in both games I'm sure.
And another option, for those of you with a decent PC like I have, you can actually have both games running at the same time. No more need to choose, just play both.