I will add...suppose Eternals and Black Widow bomb or do poorly and take the interest out of the sails of general movie goers and some comic fans. People might just say "ah I will wait until the end when we see Thor and Avengers and people I care more about". It could really hurt the MCU.

Granted, I doubt this will happen. 23 films in and really, only one mega dud in my eyes (Iron Man 3). Others, like Thor Dark World and Incredible Hulk while not great, were still watchable films and were well done. Everything else has either been good to great to fantastic. So, I have faith but I think this Phase carries a lot of risk. However, if the payoff leads to doing the FF and X-Men right, it would be huge.