Yep.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
2D sidescrollers where you mostly fight with your bare hands (but sometimes you may grab a white or fire weapon) a lot of enemies, using very simple and intuitive arcade controls and you can also walk a bit up and down and have the same camera than Renegade or Double Dragon are beat'em ups. Examples: Final Fight, the new Battletoads, Fight N Rage or Sreets of Rage 4.
3D action focused games that aren't sidescrollers and don't use the Renegade/Double Dragon camera, where you mostly fight a lot of enemies mostly with white weapons (sometimes with fire weapons or bare hands too) and have a bigger focus on fast and deep action and controls with more complex stuff that may include like parries, air combos and a huge moveset are hack & slash. Examples: Devil May Cry, Bayonetta, Ninja Gaiden.
Narrative driven games where you explore a rich and deep open world with a strong focus on its story and dialogs with NPCs but you also fight enemies on real time instead of using NES-like turn based menus, and also have other RPG elements are action rpgs (if Japanese then action JRPGs). Example: Yakuza, Judgment.
As happens in other genres, sometimes these genres get elements from other related genres. Modern beat'em ups (Streets of Rage 4, Battletoads, River City Girls...) often get stuff from hack and slash games, that is a genre that evolved from classic, old school beat'em ups. Yakuza games are action RPGs whose combat were also influenced by beat'em ups.
I'd call Batman or Spiderman action adventure open world games. Instead of being focused only on sidescrolling action like beat'em ups, they also put a lot of attention into the narrative, exploring or transversal, secondary missions like helping NPCs, upgrading or unlocking stuff, maybe puzzles or things like that: a big adventure where combat is only another part of the game.
It isn't black and white, they are separate genres with things in common, that often influence each other. As an example, some beat'em ups like River City Girls also introduce some RPG elements. Or other beat'em ups like Fight N Rage or SoR4 include many things inspired by one vs one fighting games like Street Fighter.