That's precisely what happens in every D2 strike. You've just described The Inverted Spire and The Pyramidion - fight through waves of enemies until you reach the big Hobgoblin. And if you want to masquerade mechanics as being 'hit these 4 buttons/stand on a vex plate for a certain amount of time', I could easily make the case that those 'mechanics' were in D1 strikes.
The D2 campaign was terrible too. It was the easiest FPS campaign I've ever played. It was insultingly easy and should have had difficulty settings. The final boss is a joke, frankly. Fighting Ghaul was so anticlimactic it was beyond belief. He's not even a threat in the fight. The big bad Bungie hyped up for 6 months turned out to be one of their most uninspired boss fights.
1. The Inverted Spire's boss has three different phases, each with its own strategy and arena, culminating with his weakpoint (optionally?) exploding and triggering a soft enrage of sorts; The Pyramidion's boss goes through periods of vulnerability where your team is responsible for breaking his barrier, and also culminating with an (optional?) soft enrage where he frenzies around the battlefield.
This is also to say nothing of the dynamic obstacles and navigational hazards you encounter moving through each strike.
I'm not calling either of them the best boss fight of the year, but these "phases" demand different strategies, interactions, and in many ways dynamically change the playspace. They are
inarguable improvements on vanilla D1's strikes.
2. I agree with you that, difficulty wise, D2's campaign was too easy and needed a lot more dynamic difficulty.
However Ghaul was a unique enemy with unique attacks and the player knew who he was and had a properly understood reason of why he needed to die; the big Vex in the Black Garden had none of those. I'd also consider the mission he was attached to
awesome, storming the Last City alongside other Guardians and the string section was a significant improvement over The Black Garden.
Again - to me this is a clear and definite improvement, though I'm not saying its beyond criticism.
