FFXIV dungeons are a little bit too linear?

[TouchandFeel]

My two cents as someone who has professionally designed and built dungeons, raids and other instanced content for a triple A MMO.

If you create multiple branching paths, multiple bosses that will be randomly chosen, etc. All of that is extra content and development time that must be put in and that quickly chews through development resources. From a development standpoint it is better to put those resources into stuff you know will get played, instead of stuff that may just get skipped or not bothered with.
Why have two half-assed bosses, only one of which is guaranteed to be fought, when you could have used those resources to make one much better boss?
Create a dungeon with branching paths, multiple bosses that are selected randomly and all that jazz and you just used up the development resources for creating two dungeons and so instead of two separate linear dungeons, you have a single dungeon that branches and has some slight variations in it.
While you could try to reuse or repurpose assets, they still need to be properly integrated together and things like boss scripts still need to be created, so it may save some development time but often not as much as people like to assume and many times its actually less time and effort to do something from scratch than trying to rejigger something just to reuse it.

While some people may say they enjoy labyrinthine dungeons and the like, the reality is that most players don't. Players in general really don't like to backtrack, get turned around and lost fairly easily and dislike unnecessary travel time. They pretty much want to go from point A to B and get their reward quickly while having a good time along the way. Also, single player story driven RPG dungeons and MMORPG dungeons are completely different beasts. It's like comparing a Counterstrike map to a Bioshock level, sure they are both shooters but are vastly different in most other regards.

So, long story short, wanting more variation within a dungeon (i.e. nonlinearity, random bosses, etc.) is still more and more does not magically manifest and resources must be used up to make it happen, which results in more there but less elsewhere. Add to that the general player preference for things being more straightforward and less things that seem extraneous, and more linear but refined dungeon/raid designs end up being the best decision from a development point of view.
Overall two linear dungeons actually provides more variety in gameplay than one dungeon with branching for approximately the same development resources.

I'd be all for some form of nonlinear or somewhat randomized content, but that would have to be its own thing that has a separate resource pool allocated to it.