Is money the measure of might?
Let's be honest here, Black Widow simply does not have the same buzz around it as Endgame, Far from Home, Wandavision, or Loki. Regardless of how much money it made, that does not make the film a cultural success. Avatar's the obvious example of financial success not equating to cultural success, but we can also just look at Disney's latest live action remakes, such as Aladdin or The Lion King, that have raked in the money but have been hammered by those who saw them. If we want evidence that Marvel is still doing fine, look at the success of 2/3 of their D+ shows. Not only financial hits, they're frequently discussed and praised by those who watched them. I've seen no buzz for Black Widow except by your run of the mill Marvel fans who love everything they put out. Honestly, the only comments I've really seen about Black Widow is how they butchered Taskmaster.
Anyway, Marvel's not failing (not yet, anyway), but Black Widow's opening weekend isn't evidence of the MCU's longevity post-Endgame; Wandavision and Loki, however, are.