The original "Batman" and "Superman" film series each lasted for four films, and five of the eight were extremely successfull.
It's pretty funny how you avoid addressing my actual argument. You act as though Batman And Robin and Superman IV never existed. Those are the movies that sunk the franchises, and in each case the previous outings were of markedly lower quality than the first two chapters. The superhero genre was in dire straights, and Marvel stepped in, as Out of My Vulcan Mind