I don't think developers know how to make a pure beat 'em up game anymore. It's incredibly hard to make a long game with gameplay elements that get repetitive quickly. If they don't blend RPG, puzzle, or other genre elements, the game starts to get really tiring. Consumers were a lot more accepting of games like Streets of Rage back in the 1990's because games weren't particularly long. Games have evolved and I don't think players would be accepting of such simplistic games anymore unless it's something constantly fun like Mario. I think a good example of this is the Golden Axe game that was made fore the 360 a couple years back which was hated by everyone.
Yes, but Golden Axe: Beast Rider did EVERYTHING wrong.
Maybe more beat 'em ups should be like River City Ransom, which blended beat 'em up combat with an open world environment. It was almost like a prototype of Grand Theft Auto or Yakuza. And maybe add some other elements, like give playable characters the option to equip weapons or carry healing objects. It could work for franchises like Final Fight or Streets of Rage, right? :)