I think the real question is: The decline of what?
If these 4 movies are comparable in quality to their predecessor or better, then:
Black Panther 2 is doing a billy.
Captain Marvel 2 is doing a billy.
GotG 3 is doing 850+ million.
Dr. Strange 2 is doing 670-700+ million.
Hell, even if you feel BP 2 or CM 2 wouldn't be as "successful": if CM 2 makes 200 million less at the box office... that's still more money than any Thor movie (including Ragnorak), Homecoming, any GotG movie, and 2 out of the 3 Cap and IM movies. Even if BP2 makes 500 million less than BP 1, that's still more than all the aforementioned movies as well.
So with the aforementioned movies in their back pocket for the next 2-3 years... I mean,
what if Shang Chi and
Black Widow only do "Ant-Man" numbers (while having an Ant-Man budget)?
The internet seems more invested about this downfall narrative than the general public does. Ironically they're all gonna go bat shyt crazy at comic con regardless.