Every mmo will go downhill, there is just a few factors that it depends on.

1. Other mmos currently on market
2. Their engine, its vastly difficult to make big changes to a game with decades of content that may break if you change your engine.
3. Effort by publisher

1: I think we can agree that right now, the mmo market is not ... amazing, yea there's a few games that can keep you busy but its not on the same scale as XIV at this moment, however a few at least promising mmos are on the horizon.
2: They engine is fine for now, graphically good for another 5-10 years (if you push it), this does not mean that in 5-10 years its considered "ugly" but newer games will outshine it vastly.
3: This is a bit of sour point for me as I feel like SE realised their current market share and is not going all out anymore, content scrapped or cut short, longer cycles.

XIV will go absolutely fine for another 5-10 years unless a new MMO could confidently prove their a valid alternative, if that happens it is possible for a spiral to happen like in wow, most do not want to play an mmo that they consider dyeing, even if an mmo cant really "die" even FFXI is still around and not dead.

XIV is a insane example of how you can reduce content but also increase the quality, which leaves many unhappy about the missing content but many really happy the quality ... and yes ... that quality is mostly of visual nature, a lot of the content is designed with the idea that its dead in 4 months which means its always going to be by the numbers.

It is what it is, in a perfect would you would have 2-3 mmos that are constantly fighting over the crown, one upping each other.