Last time I ran dungeons in WoW, they were also pretty linear. Some do allow for doing some bosses in a random order, but many are just as linear. I also don't see them as being any more or any less deep - you have trash to deal with, occasionally with a mechanic that needs to be handled, and then bosses with 3-4 major mechanics.
Not sure where you're getting that dungeons in FFXIV are identical in layout either - pretty sure each map is different. Often linear (as often as WoW's are, especially post-Vanilla content - the ones from the classic game are the only ones that really had the open variation, but that kind of design has been missing from new dungeons for about 11 years now), but that's nothing new for either MMO.
Speaking of identical - funny that of the two games, only one offers dungeons that are the "hard mode" of previous versions that are literally the same location, with the exact same bosses (occasionally you get that one extra tucked in a corner) and trash, and where everything just now hits harder and bosses maybe get a new ability or two...versus reimaginings of the location into a sort of "sequel" version of the dungeon that often leads to new spots on the map and has brand new bosses.
That all being said, you've missed the context of my comment, which was related to the amount of content (given that the person I was quoting specifically mocked what they saw as a decline with the suggestion that we'd be getting one new dungeon and one fight in savage, it's exceptionally clear they are talking about amount). WoW drops something like 5-7 with the expansion, keeps them around with additional difficulty modes (that are, again, the same dungeon with boosted enemies - they use that to great effect with their mythic system, but that's not what is being discussed - remember, the comment was about decline in amount, trying to switch it to a subjective discussion of quality just confuses the issue), and if those players are lucky they'll get 1-2 more in another patch before the 8 or so month lull before the next expansion (in a good year, which they've struggled to get back to after multiple year+ gaps).
As for the UE4 upgrade comment - I admit that I did undersell the effort on the part of the developer needed, but that was in response to what I feel was an overselling on your part. Bringing up that they are updating to a new engine, in a thread where talking about making upgrades to a game are common, and NOT clarifying that it's not an in-house engine and that another developer made the engine they are working with (in both cases, UE3 and UE4) implies that SQEX is holding back on us by not doing something similar.
When they'd have to redo their own engine/make a new one AND redo the code, you have a situation that isn't nearly as comparable as you seem to want to imply.