That argument only makes sense if you've downed a fifth of Jack and snorted a couple of grams a coke - or if you're completely and totally ignorant of the law.
And yet it made sense to me, and I'm sober as a judge for some reason.
There is no mechanism I am aware of for a copyrighted work to become generic. It can be formally released into the public domain, but that's another issue and is not applicable here as no such formal release has been made.
Well, works can also enter the public domain through other mechanisms, such as most famously having the copyright term expire.
But the earlier poster didn't say that they might become generic, he said that they might be generic. This would be the scenes a faire doctrine. I haven't watched the show, but I recall that it's a 5-person team, so I'd bet that they fall neatly into what TV Tropes calls the Five-Man Band:
The Leader -- The leader of the group. Can be a mastermind, charismatic, levelheaded, headstrong, or some combination of the four. Often also The Hero.
The Lancer -- Usually a contrast to The Leader. If the Leader is clean-cut and/or uptight, the Lancer is a grizzled Anti-Hero or Deadpan Snarker; if the Leader is driven and somewhat amoral, the Lancer is more relaxed and level-headed.
The Smart Guy -- The physically weak, but intelligent or clever member. Often nerdy and awkwardly played for comic relief. Sometimes unconventionally young (early- to mid-teens). Sometimes a Trickster and a buddy of the Big Guy.
The Big Guy -- The strongman of the team. May be dumb. Or mute.
The Chick -- A peacekeeping role to balance out the other members' aggression, bringing them to a nice or at least manageable medium. The Chick is often considered the heart of the group. This role is played by a woman or girl. Someone female. Otherwise, it is not a Five-Man Band.
This is a stock description of characters. Anyone is entitled to use it, regardless of whether the first work to use it is still copyrighted, and regardless of whether people are copying it from other copyrighted works. It's like how everyone is entitled to use a spooky eastern european castle, with moon-baying wolves, in a movie about vampires.
Now, if the Power Rangers characters are more defined than this, and if the use involved their more defined, non-generic character traits, that could be an infringement. But if Power Rangers doesn't do much character development and is more like a modern version of commedia dell'arte, where it's just about how the lovers will enlist the help of the servants to marry despite the wishes of their parents, and so on, but with robots and karate, the characters might well be considered generic.