Whilst I definitely agree with you, there's something else to bear in mind here. Those 4 jobs did get added complexity, but they also got big changes in terms of the feel and fun-factor of the class. YMMV of course, but "added complexity" isnt the only thing at work here.
Every class generally has a focus to their rotation. They all have 5-6 regular buttons they press in either a fixed rotation or a priority system, but they all have something else to manage. Monk is about managing the 3 stacks of GL. Ninja is mostly about Huton uptime and regular Ninjutsu use. Dragoon was mostly about weaving lots of OGCD hits in between an otherwise rather fixed rotation, but now suddenly it also has the management of the BotD buff to contend with, which adds the additional frustration of "oh no my buff fell off due to a weird boss mechanic". This frustration is something Monks had to deal with pre-HW, but DRGs largely didn't.
BLMs got a massive change where instead of just watching their mana and reacting to procs, they now have a very very fixed rotation that harshly punishes them if they're even half a second late hitting that Blizzard 4. Bards didn't really get a complexity change but suddenly they're a caster and can't move as much during their rotations.
Any player forced to play one of those classes would easily adapt and learn the new rotation, but the fundamental feel of those classes has changed massively. The additional rotation skills add in layers of focus and frustration that are far beyond what players of those classes experienced in 2.55, whereas Monk/Ninja by and large got skills that REMOVE frustration by helping their buff upkeep.
I can certainly see (and know a lot of people like this) DRG, BRD and BLM players just leaving the classes in droves not because it's too hard but because it no longer feels like the class they loved playing back in 2.55. It may be that the community buzz about how fun tanks are (particularly all the fell cleave memes at the start of HW) has made these previously dps-centered players at least dabble in tanking.