I think it would depend on a couple things for me.

First, is 14 going to continue development or is it just going to be stuck in maintenance mode? If EW really was the last expansion and the game wasn't going to be developed further, wasn't going to receive new content, etc. then I wouldn't keep playing 14 regardless of what was going on with other games. I might check back in once in a while and if there was anything left I really wanted to do at the time I'd still do it, but I wouldn't, say, start levelling a new job or start grinding for a new relic. My general ongoing play of the game would stop once it was "done."

Second, is 17 a good and interesting game? I'd be likely to pay attention to 17 and would absolutely be willing to give it a shot, but I wouldn't say that I would absolutely play it without having the experience. I didn't play FF11 much more than an hour or so because I just didn't like it. It didn't matter that others do like it and/or because it's a FF game.

So, yeah, if 14 ended and 17 took over as "The FF MMO" it could conceivably result in me not playing either of them.