It's perhaps not a defining aspect of cinema as traditionally understood but the fact that MCU requires deep immersive knowledge does make it narratively interesting (true also of the comics). It's a new type of narrative, one critics have a hard time grappling with.
Perhaps post-Scorsese, we can movie the debate forward by stipulating that Marvel Cinematic Universe movies are not cinema as traditionally understood and ask "well, what are they then?"
Here's a stab at a definition: MCU shows & films make up a multi-platform open-ended multi-authored roman fleuve whose appeal comes from immersion in the density of world-building.
Now, I personally have never liked the open-ended soap opera aspect of Marvel comics (and later MCU films), preferring to skim them for surface pleasure to deep immersion. But a proper critical account needs to explain the appeal to the non-skimmers, the deep fans.
Who will be the Manny Farber of Marvel comics & MCU films -- the critic who can burrow deep into the form and explain its unique properties and distinctive pleasures? I'm hoping @douglaswolk does that in his forthcoming book.