Now that we have a working definition of combat, we can start talking about subsets. Let's first narrow our scope to action (i.e. real-time) melee-centric combat. Punching and swords. We're going to talk about some different types of game that focus on this gameplay.
Before we get in-depth about the relationship between the arcade sideways walker (ASW) and contemporary 3D action, we need to understand a third subset of combat games, one which has existed continuously from the late '80s to the present: the brawler, aka the beat-em-up. To connect the ASW to modern 3D action, we're going to show that both contrast identically with brawlers, i.e., they are both not brawlers, in the same ways. Yeah, it's a bit weird and roundabout, but you'll pick it up as we move on. Or you won't. It doesn't really matter - I'm still writing.
Once more, for clarity's sake, let's specify the three categories of game we're discussing:
2.) Arcade sideways walker (ASW). Lifespan: 1988-1993. Highlights: Altered Beast, The Legendary Axe, Magic Sword. Elucidated previously
3.) Modern 3D action (M3A). Lifespan: 2001-present. Highlights: Devil May Cry, God of War, Muramasa: The Demon Blade
1.) Brawler (B). Lifespan: 1986-present. Highlights: Final Fight, Guardian Heroes, Shank
The defining elements of all three of these game types are opposition and goals. The opposition in combat is an autonomous collection of vulnerable threats. That leaves plenty of room for variety, and the different approaches to this (the enemy characteristics, as it were) will individuate these styles. Brawler enemies are particularly easy to recognize, allowing the games to be identified without external or superficial trends. I'm intentionally avoiding the all-too-common dismissive description of "games like Final Fight", just as I avoided classifying platformers as "games with jumping" - those inductive definitions rely on convention rather than inherent qualities; they're the equivalent of categorizing action movies as "movies starring Bruce Willis".
The first question I ask myself when determining whether a game is a brawler is: do the enemies function individually? "Well-rounded" may be a bit of an exaggeration, but the idea is that each foe has multiple patterns of attack that can be selected situationally. For instance, the geishas in Double Dragon Neon will pepper Bimmy with paper fans if the player keeps their distance, but use a quick slashing attack at short range. In the original Double Dragon, the most basic enemy is William. William can be equipped with multiple weapons, which he will utilize accordingly, or even pick one up off the ground to change his attack pattern.. He can punch, jump-kick, and even uppercut the player. In the simplest possible scenario, such as River City Ransom, enemies may be limited to two behaviors: attack and guard. Other combat action games may very well use multi-strategy enemies, but they more heavily rely upon unitasking foes, allowing a player to evaluate an encounter based on the population alone. We'll get into this further when we move on to modern 3D action. For the time being, let it suffice to say that the most basic enemy in DmC simply tracks the player and attacks at a fixed interval.
The result of the brawler practice is that the player is required to determine their strategy on the fly. The enemies have multiple states, and before executing an attack, the player needs to evaluate the state of his target. This needs to be done for every attack. The player can also take advantage of this: if he knows the parameters that determine enemy state, he can actively manipulate it through his own behavior, continuously consciously engaging the opposition. It's a generalized notion of countering, beyond the modern reliance on insta-counter buttons (Arkham Asylum).
If fighting a same enemy twice can be a unique experience at the whim of the game, then we're pretty comfortably in brawler territory. These games are about one-on-one battles - though the player may be controlling a large space occupied by numerous foes, he targets his opponents one at a time, even if it is just to grab a thug and bowl him into his incoming buddies. The focus is on those aggressive engagements, not on staying alive or perpetuating some meta-game score (combo counters etc.). This is one case where the game really is all about killing (or knocking out, I guess). Why do you think almost every brawler respawns the player immediately where he died, and locks progress outside of each arena? This reinforces that the only way to lose is failure to kill everything.
It's somewhat misleading to say that the classic-style beat-em-up is past its prime. The classic-style FPS (DOOM 2, Aliens of Gold) is all but dead too - that doesn't mean the genre is. Retro throwbacks like Castle Crashers and Scott Pilgrim vs. the World can be a treat, but also serve the delusion that the brawler has become less popular because it failed to evolve. In fact, it's evolved so far that many have forgotten the lineage. You may be surprised how similarly you interact with No More Heroes and Turtles in Time, or Assassin's Creed 2 and Knights of the Round.
Aw here's a weird thought. Is Pokemon the RPG version of a brawler?