That is true, but not totally linked to Marvel. Folks have soured over time on comics in general for various reasons. Including many at one point using them as a speculative market whose bubble burst.
Quality is mixed for sure but even good quality books now are lucky to reach a fraction of what they were at a peak. Nature of the beast by default.
Other factors can decrease them further. I am a little surprised that there are not more folks reading them with success of the films, but it is what it is I guess. I know I have a fair number of friends that are new into comics and tend to shy away from the Marvel and DC stuff and more into Indy things.
I agree it isn't limited to Marvel. The average person doesn't read books anymore. And I'm not surprised the movies haven't generated additional sales. Movies are perfect, in 2 hours the whole story is complete. Compare that to waiting month's, or even years, to see a comic book storyline come to fruition.