Batman, Superman, Wonder Woman. They change with the times a little, they may get a little muddled times - Batman breaks his back and hands the mantle to Azrael; Superman gets a new lightning-themed costume and set of powers; Wonder Woman goes blind, loses her royal position, whatever - but they're characters, icons, concepts even, that remain staples of American popular culture and conciousness through and through.
Power Rangers? I still wonder how and why they keep going. I know the annual new sentai shows give them plenty of cheap, pre-made footage to use, but I'm still incredulous that Power Rangers itself keeps getting renewed year after year.
Besides, compare the two. Someone of any age, any time can wear a t-shirt, a belt buckle, a tie, a hat with the "S" shield on it and it's a timeless symbol. I challenge anyone over the age of 10 to do so with anything Power Rangers-related.
In the great, unsifted ether of history, I'm glad that Bruce Timm and others decided to put their creative energies into something like the DCAU rather than Power Rangers.