Lets use World of Warcraft as an example, since it is still the 'best' mmo out there of this genre.
To be honest PVE wise, once you have everything on farm in WoW, you don't need to log on other than raid nights and it is currently the same situation in FFXIV. Whats different is, every 'raid' in WoW have somewhat 10 or more bosses, and even if you have everything on farm it takes couple of raid nights. We currently have 3 bosses to do (not counting t5), and obviously it doesn't take that long. So all in all, i think there is a lack in boss numbers.
I really hope they make the new dungeon a raid on par with coil, since the mechanics design of the dungeon has already been done, tunning/adding mechanics to be raid viable would be much more achievable than starting something from scratch. We also have 2 hardmodes coming out for tome farming so I really dont think if it was a normal dungeon it would add much depth to the overall PvE gameplay.
