Rome was not destroyed in a day. If DCEU could somehow survive from BVS & JL for a while, just one single mixed or bad work like MOM, Thor 4 couldn't hurt MCU fundamentally.
The problem is Marvel Studios haven't deliver something excellent for a long time. You can argue No Way Home is pretty decent, but everybody know it's based on the characters we loved and not the norm of this franchise. GOTG3 performed good, but it worked more like Wonder Woman or Aquaman, earning studios more time of fixing.
The Marvels is unlucky to be released at this point. Many haters, maybe even some presses, will blame Brie Larson and ignore the real problem inside the current Marvel production mode. I wish later MCU films also flop hard so people would know it's not her fault.