I don't really think you can judge an MMO's content by the number of people watching streams. As someone else mentioned, there are a lot of reasons streamers' views might drop off.
Honestly I can't see how watching someone play XIV could be fun anyway. Even for PVP or raiding, that just isn't where its entertainment value lies.

We don't even have all of the content for Stormblood yet. We're getting a new 24-man raid and a new deep dungeon among other things, it's not like there's no new content.
Granted all of this is stuff that's been done before, but can you really blame SE for putting their development hours into things that they know have worked well previously? Trying to break the mould can be expensive, and the exciting new content you create can end up being a spectacular failure and a huge waste of development time.

Sure, I wish SE would try more new stuff, but tbh I enjoy this game for what it is; a linear storyline with fun stuff on the side.