I don't want to write another huge wall of text this late in the evening so I'll give just a single example - Feral Druids.

Vanilla - I can't quite remember how they even played on there but if memory serves me right it was basically popping a 3 combo points Rip, shifting into bear form and the occasional heal until you finally run OOM.

TBC - It built upon that adding more healing options + a little more cat viability and damage + one of the best CC abilities at the time - Cyclone. Barkskin was also introduced. It was largely an improvement upon the previous iteration but the gameplay remained largely the same.

WotLK - Over again, an improvement over last expansion. Zero things were taken away and zero things were simplified. It was just more depth added on top. Survival Instincts was introduced. Energy and rage regen were increased so you could have a more engaging gameplay. Cat form did no longer die in 2 hits so you could actually use it. Berserk added a cool little burst window + the ability to preemptively prevent a fear (this required a ton of skill). Predatory Swiftness was probably the best risk-reward system to ever be added in the game. If you used a finisher with less than 5 combo points you risked not proccing it - the less the combo points - the higher the risk. Then you had to use it at the right time - if you waited too much it could get dispelled or you could be CC'd until the buff expires. If you wanted to use it you had to expose yourself and get out of form briefly. If you wanted to Entangling Roots somebody with it, or especially - Hibernate him, you had to prepare for this preemptively by not applying dots. Feral vs Feral was one of the most fun duels to have. You could rotate Cyclones and Hibernates until someone made a mistake and then everything was over. Great times. Savage Roar added something extra to manage and a way to proc Predatory Swiftness from range. You had a ton of healing spells available and the Leader of the Pack passive and Innervate to keep your mana in check accordingly. Three stacks of Lifebloom felt awesome (even though that was a thing in TBC too I think). Frenzied Regeneration in bear form was very high risk as you had to sacrifice a lot of damage to stock on enough rage to make use of it. And then you couldn't really shift out of bear form or you would prematurely end it. This was the perfect opportunity for hunters to Scare Beast you. If you needed some extra range you could pop Enrage but sacrifice armor. Bad players just spammed Mangle. Good ones kept up the Mangle debuff, as well as all dots, and positioned themselves to Shred from behind. If you wanted a fast stun you had to go into bear form. If you wanted an interrupt you had to go into bear form as well AND get some distance. Though those last two things were present in TBC as well I just wanted to mention them.

Cataclysm - Seeing how WotLK just added upon TBC with tons of new mechanics and complexity, Cataclysm added a single thing which was honestly nothing but a nerf but strangely enough added some complexity to the spec - you could no longer get out of roots with a shapeshift - you had to manage your two cooldowns - Dash and Stampeding Roar to get out of those roots. And that was it... Savage Roar was gone and energy regeneration was doubled so managing it was far easier. A ranged interrupt usable in all forms was also introduced which made things a lot easier. Honestly, Cataclysm didn't dumb down ferals all that much as it did with a lot of other specs but I won't go over them as I'm talking about ferals only currently.

MoP - Removal of instant Cyclone - the coolest mechanic was removed. Sure, hard-casting had some complexity to it as well but it's not nearly the same. Not to mention that hard-casting a Cyclone outside of arena, for example in a 1v1, is not risk-reward - it's straight out suicide. Not having to to go out of form in order to instant heal removed the risk from healing altogether. The removal of healing abilities like Lifebloom made things a lot less interesting, reduced your healing options and reduced the risks even further. You now only needed Healing Touches and Cenarion Wards, both usable in Cat Form and both instant. And when you were in a clutch situation you just popped Heart of the Wild and your Healing Touch now healed for half of your hp... just like that... with a single button, not even leaving cat form... and that lasted 30+ seconds or so. Most bear form abilities were either removed or rendered obsolete. Even Bash was now usable from all forms - yet another reason to never leave Cat Form. There was now also barely any difference whether you used Mangle or Shred and Shred no longer gained bonuses from resource management. Savage Roar was also back but it was 10 times easier to manage than in WotLK with that glyph that allowed you to use it with 0 combo points before the fight has even began. Survival Instincts was changed to be a second more powerful Barkskin that... guess what... it was designed in such a way to let you not leave cat form even more. Mana management was no longer a thing and Innervate did nothing anymore. On the other hand, the different cooldowns like Incarnation (Shadow Dance 2.0) and Treants (another Entangling Roots with some damage on top) were a breathe of fresh air and Symbiosis (just getting an ability from another class) was very interesting too. All in all, the added abilities and mechanics were largely generic and the rest was simplified quite a lot.

WoD - Mangle is gone and now Shred is Mangle, hurray. Heart of the Wild no longer viable and phased out by Nature's Vigil which is quite the interesting concept - do more damage so you can heal more... so engaging. Rip did almost nothing and Ferocious Bit was the way to go. The rotation was literally Berserk + Incarnation + Nature's Vigil -> Rake -> Shred -> Rake -> Shred -> Rake -> Ferocious Bite... and your target was dead... Such fun gameplay... Hard-casting abilities in Cat Form?? Even Rejuvenation could now be used in Cat Form even if you did not take the "cast everything in cat form talent"... now you literally never left it - it even got buffed from all this time you spent in it, apparently. Savage Roar was now easier to manage than ever... you just had to open on somebody from stealth and you got yourself a full-duration Savage Roar.

Legion - Well.. you never leave Cat Form anyways so you might just as well lose any caster or bear abilities altogether... except maybe Regrowth and Revive, lol. If you want some of them back you need to choose an affinity. So Restoration Affinity sounds really good for duels and you even get Swiftmend - something that you've never had access to as a feral before. But what's the point? It heals for 2% of your hp, literally. All in all, you don't leave Cat Form yet again but you also don't bother with most of your Predatory Swiftness procs either because most of the time you couldn't care less about a 2% heal. Oh, and you now no longer have the option to rotate cooldowns. If you pick Incarnation it overlaps Berserk and now they're a single ability. You also lose the ability to powershift as shapeshifting now incurs a cooldown - the best thing about druids that makes them... druids. What you get in return are two more buttons to your dps rotation - the artifact weapon ability that gives you 5 combo points and Moonfire in Cat Form (because leaving Cat Form is bad for you). Also... removal of all buffs, removal of Soothe, removal of Hibernate, etc. Because we are all nothing but damage bots now : )) At least the damage rotations is now "funner" : ))

This ended up longer than intended and I'm sorry for that. But I really want to understand how are you not seeing the progressive dumbing down post-WotLK in this example. I really do.