Except those 35 bosses cost as much development time as a single dungeon, and is hours and hours of new content to deal with. While a dungeon is a 25 minute new experience.
The statement that those 35 bosses take as much time to develop as a single dungeon is entirely unsupportable. Its not hours and hours of new content to deal with, its 35 boss battles to deal with and then run again and again and again. There were, what, eight new dungeons in Heavensward? So around 24 bosses, with the "shortfall" from 35 accounting for the rest of the dungeon development time. I dont think theres a meaningful difference in development time (nor in completion time), though thats as equally unsupportable as your claim. I also still fail to see how fighting the same 35 bosses again and again is either superior or meaningfully different from running the same nine or ten dungeons again and again.
Edit:
PW boss chain (or Zeni NMs) was just a small part of an expansion / patch. It's comparable to one end game dungeon or one EX battle.
Again, I feel this is an entirely unsupportable statement, if you have some source for the dev team being either wildly different or in the same ballpark I'd love to see it,