It's not so much about the reception to Quantumania or any other past MCU film. It's about the quality of those projects (which obviously correlates to the reception) and the lack of faith in the creatives behind those projects going forward.
Now I realize none of Waldron, Loveness or Waititi are involved with F4 but the stink comes from Marvel Studios in general. I enjoyed Wandavision but I know I'm not putting a lot of faith in Matt Shakman to deliver here. As for the writers, I don't know anything about Jeff Kaplan and Ian Springer but again I'm not exactly inclined to give them the benefit of the doubt at this point. We can only hope they become another Markus and McFeely.