To play devil's advocate on Man of Steel, in terms of box office it did better than nearly all inaugural films (in some cases also the first several films) of other modern era superhero franchises since 2000. The only exceptions to that rule are Spider-Man and Venom.
Adjusted for inflation since that is the only fair way to compare them. These figures are worldwide gross:
Man of Steel (2013) - $735,725,572 in today's dollars
X-men (2000) - $441,511,477 in today's dollars
X-2: X-Men United (2003) - $568,487,280 in today's dollars
X-Men: The Last Stand (2006) - $584,585,162 in today's dollars
Spider-Man (2003) - $1,145,738,600 in today's dollars
Fantastic Four (2005) - $434,269,695 in today's dollars
Batman Begins (2005) - $491,596,489 in today's dollars
Iron Man (2008) - $697,301,607 in today's dollars
The Incredible Hulk (2008) - $313,903,873 in today's dollars
Captain America: The First Avenger (2011) - $422,659,655 in today's dollars
Thor (2011) - $512,487,112 in today's dollars
X-Men: First Class (2011) - $403,332,005 in today's dollars
Fant4stic Four (2015) - $181,826,603 in today's dollars
Venom (2018) - $874,668,942 in today's dollars
And although not necessarily intended to be franchise starters, had they performed well they likely would have been:
Green Lantern (2011) - $250,754,991 in today's dollars
Superman Returns (2006) - $497,693,494 in today's dollars