I would try it out for sure, unless initial reviews paint it as being like, 1.0 levels of bad.

Would I drop FFXIV? I'd only fully drop FFXIV if I stop enjoying it. I'd just play it less if there was another MMO in my life.

Whether that would happen depends on FFXVII. Do I like it? Is it just 'FFXIV but better' or does it have its own distinct game design philosphy and focus? There's no way to really answer this question without seeing this theoretical FFXVII Online firsthand.