Sunken Temple was relatively fast, and for all its odd-ball design, fairly straight-forward. Across leveling 5 classes through there back in Vanilla, I don't think I've ever had a party break up in there. Even Gnomeragon would probably be a better example.
Maraudon likewise had 5 core bosses and 3 optionals, though because of the backtracking or easy exit after the 3 optionals, it was often split into "Purple" or "Green" runs for those with less time. It would have been a pretty much standard dungeon if not for that side-path being longer than usual. In terms of basic layout and time... it was a lot like RfC or BfD, just... higher level.
Blackrock Depths, on the other hand, had 20 bosses. It was quite simply a mega-dungeon, and you could work many different more reasonably timed paths to grab 6-12 if people didn't have the time to do the whole thing. And that was fine. There's nothing wrong with signing up to do just one wing or just the majority of BRD. At least, it's no more an issue than if someone signed up to do a Legion leveling dungeon (with no more bosses in total than BFD had in itself), and only did one of them. It's cool that you have more bosses and time available within the same art assets in case someone wants to do an extended run.
That's a simple efficiency-minded feature that provides a unique experience, more so than any overall failing. The only failing in that is that the less efficiently chartable bosses and wings for a quick run will be seen less often, and are therefore less deserving of development time unless the pay-off of that unique experience is sufficiently high. (Granted, given that BRD is still probably the most famous WoW dungeons, among veterans and non-WoWers alike... it probably was. But that's obviously have increasingly less novelty value with each dungeon to follow suit.) But it wasn't the size of BRD that most discouraged players. It was how far one had to travel just to get there, through zones that were considered among the least aesthetically interesting in the game (Searing Gorge and Burning Steppes) and were therefore hard to general (think "shout") chat a group out of players out of, pre-LFG chat.
And for the record, 1.0 Thousand Maws of Toto-rak was almost identical except in that it had a 30-minute time-limit (Aether poisoning or the like), it lacked the slime and diremite traps to forcibly slow you down, and had 2 optional bosses. However, though these optional bosses were removed, their rooms are identical; their loot-chests were simply replaced with... empty chests. Now, rather than being able to finish via any of the three diremite bosses, you could now leave only through Graffias's lair (one of the weaker two, but furthest away from the entrance), actually making the path to relative completion... less intuitive than before. The previous experience was that you burned through the first 2 bosses, got to where you could reach any of the diremites, and did as many diremite bosses as you could will still ensuring you had time to escape. Now, you... do a normal dungeon, but with lots of now-useless side-paths... and slime to slow you down. That's more an "even now" issue than a "because we explicitly changed it to make it slower and less rewarding" issue.
@Ultimate. Agreed. As few people as might go through Ultimate, we probably all benefited in some way from its existence, if only via the influx of subscribing players. Now we just need content that can allow us to keep subscribers between major patches without going the semi-recent WoW permagrind route.