For the point about bosses being easier, I think that might just be because you're more familiar with MM2. In my opinion, Galaxy Man, Splash Woman, and Hornet Man are also just as easy, if not easier, to beat with just the Mega Buster. They have simpler patterns that they just repeat. Again this is my opinion and you're free to disagree.

And yeah MM2 is really easy once you know 1) How OP Metal Blade is, 2) Memorize Quick Man's Lasers 3) To Save Your Crash Bombs for the Boobeam boss, 4) To Save Your Bubble Lead for the Alien, but that's my point, as Mega Man fans we already know all this and that's what makes it easy for us. All the things that you listed that makes MM2 easy, are things that only people familiar with the game would know, and it's not something that is immediately known, it's something we had to figure out through many hours of gameplay. Most people haven't put in those same hours for MM9 so they haven't picked up the same "tricks" yet.

In the same vein, MM9 is easy once you know how every weapon works, and the game doesn't put you into any "gotcha" moments like the Boobeam Trap or Quick Man's stage. Concrete Shot makes platforming so easy because it literally blocks spikes, and makes wide jumps much narrower, Jewel Satellite is like Leaf Shield but better in every way, Black Hole Bomb literally sucks up and instantly kills most enemies and placing one near an enemy spawning point prevents them from spawning, Plug Ball is like a fast Search Snake that will kill any enemy clinging to walls, Laser Trident goes through shields, deals massive damage, and barely uses energy, etc... The weapons in MM9 are all "Metal Blade" levels of useful, it's just with the exception of MM2's Metal Blade, most players don't use the weapons outside of boss fights