Actually the big bang theory is not falling apart. It has just become incorporated into a larger theory. The Big bang theory was proposed because it explains the observable evidence (cosmic background radiation and expansion of the universe) The big bang theory only goes back to the singularity but could not explain what happened before. The multiple universe theory still says the big bang happened but attempts to explain what happened before it. Not saying that it is correct by the way but it does fill in a lot of gaps left by other theories, such as why is gravity as weak a force as it is in the universe.
Is the universe expanding? There’s some serious questions about that. Understanding of the red shift has always been sketchy.
I thought the big bang theory was falling out of favor because of the new evidence that the physical position and movement of some heavily bodies contradict a model with all matter generated from a midpoint. Eg, we have discovered heavenly bodies that are aligned in long walls among themselves that defy an alignment and motion that suggests they came from one centerpoint.
I thought that is why the big bang theory is becoming outdated. I though the scholars now can’t support the theory that all matter was propelled from one centerpoint radially, uniformly in all directions, as the origional big-bang theory would suggest.