The problem with declining camera sales is not due to sensor format.
It is due to replacement technology. ie. Mobile Phone.
Unless you are a serious hobbyist or professional, the latest Mobile Phone can probably take better pictures than a compact or Mirrorless DSLRs if all you need is a 12mp picture to capture moments or share on social media.
Phones better than compact or mirrorless camera? Not true leh..according to Robin Wong of the latest samsung note 20 ultra for example. Yes the main camera/lens is good ( 108mp sampled down to 12mp) but for the ultra wide lens or telephoto lens it sucked big time.Just gimmicks, sure if you need a fast pic it can suffice. The tele lens is a folded periscope design. I remember as school boy it was a fun thing to construct but nowhere near a submarine periscope. I guess it was one way to get around the space constraint so until real better lenses are designed this multi lens thing has a long way to go.