Well, for a new player like me there are plenty of reasons.
If they announced tomorrow that support has ended, I'd estimate that I still have a few thousand hours of things to do in this game. I have 250 hours /playtime already, and I am only level 71 on my first job, I've done no side-quests (only MSQ), I haven't even begun to collect minions / mounts / triad cards. I haven't started completing any logs. There's housing, glamours, RP ... and I haven't even joined an FC yet. I don't play this game because of the promise of development down the road. I play it for what fun it can offer me now, today.
So, in regards to the question the OP posed... I'd be torn. I'd want to "finish" XIV which would take me some thousands of hours, at least. Yet, I'd really want to get into XVII.
When it came down to it, I suppose I'd switch to XVII primarily, and play XIV during downtime / content droughts. /shrug