Yes, they are smaller but are 2D sidescrollers where you can walk a bit vertically in addition to horizontally. Games like Bayonetta aren't sidescrollers and aren't 2D. And their gameplay, even if evolved from beat'em ups, it's way different.
It's like to say Halo and Doom Eternal are the same than Contra or Gunstar Heroes because you run and jump while constantly shooting. No, they are very different games even if a genre evolved from the other. Beyond the switch from 2D to 3D, the gameplay and progress through the game changed a lot.
Yeah, I agree with you in some areas. Sure the 3D aspect can create many new styles of gameplay, while original Crash Bandicoot and Mario 3D World translate more directly the more classic 2D vibe but with the z-axis, there are also games like Mario 64/Sunshine and Banjo Kazooie that have a whole different approach to 3D Platformer.
I also agree with you that people want to create too many dumb and unnecessary sub-genre names like character/stylish/majumbo jambo action that nobody uses.
But if you take Beat n up strictly to 2D side-scrolling games you also wouldn't call Yakuza games beat n ups. Their progressions system with multiple unlockable styles, movesets, new weapons and the sheer quantity of moves Kiryu has at the start of the first PS2 Yakuza game is more than most 16 bit games we have in the genre. Yakuza games also don't feature a jump button, something that was crucial in most Final Fight/Streets of Rage games.
Most of the Yakuza 0 gameplay ideas also come from its spin-off Ryuga ga gotoku Ishin that is more about guns and swords.
Batman Arkham games you also unlock new moves, have a grading system after each fight encouraging variety (Origins and Knight mainly) and even some interesting combos.
Ryse son of Rome also uses Arkham's combat but the game does not have any new moves or weapons and it clocks in after 4 hours.
Marvel's Spider-man PS4 also has a more intricate combat with aerial emphasis web shoots that keep enemies in the air and lots of unlockable moves and gadgets. (Not to mention Web of Shadows that is DMC levels of crazy combat)
So which one of those are 3D beat n ups?! You know I don't mind just criticizing and appreciating each style of combat.
Even Streets of Rage 4 uses a complex juggling system, a void/training mode like DMC5/Bayonetta, frame-specific techs, launchers and a proper grade rank system to each level.
But I really appreciate your answer and contribution to this discussion, it's nice to talk to people that like the same stuff as me