Marvel has abandoned character development that made their heroes icons of heroism and bravery and instead focused on narcissistic characters that generally don't grow or improve and who are defined solely by their physical powers, and as a result they've lost the moral soul of what their stories used to be all about.
Unfortunately, this is ultimately also a problem of their source material in the comics, which -- like most everything else over the last 15 years or so -- became obsessed with morally relativist post-modernism, "deconstructing" heroes, and rejecting character development in favor of snarkily showcasing power and blurring the lines between good and evil.
Villains are being rehabilitated (without actually having to atone for their past actions) and heroes are being dismantled or outright replaced with knock-offs who have less heart and personality.
The horrendous box office projections for The Marvels is just Marvel reaping what it's spent the last decade or so sowing.