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
I did address your argument. Your argument is to ignore the huge successes of both franchises in order to focus on the ONE FILM in each series that did poorly.(and actually, "batman and robin" STILL made $107 million domestically)
And as I pointed out, ignoring the successes to focus on the failures is not a fair way of looking at it.