You could have argued that a bit earlier because you were right that the products coming out were bad.
But Thunderbolts and Fantastic Four were well recieved. Audience and Critics both gave them very positive praise, they were not "bad movies". And they did pretty well at the box office....just not well enough for the crazy high budgets every marvel movie gets nowadays.
Superman made money, but it didn't make anywhere near the avenger movies in their heyday.
So yes....its fatigue. Greatest Marvel movie ever made could come out right now, it still won't do the numbers of old school marvel products. Deadpool and Spiderman are the exceptions, people just really like those products regardless of the rest of teh MCU.