Cav's clearly on Demare's wheel
Yes and no... A few moments before the crash, Cav was very close behind Demare, but it looked to me like Demare was pulling away from him. Never mind the slow motion replays, watching it live as it happened, it was clear that Cav was losing ground, and therefore out of the race even before the crash.
Now, I don't know how close you have to be at that speed for it to count as being 'on the wheel', nor how much of a gap there has to be before it's considered enough space for another rider to move into it, but it was apparently enough of a gap that Sagan didn't see him there when he started to jump across onto Demare's wheel.
As Cav and Sagan came together, there was a bit of a lean from Cav and an elbow out from Sagan. I didn't see any malice in it, just the usual sprinters' argy-bargy with riders fighting for position. Had they not been hard up against the barriers, it might not have ended so badly for Cav. (compare and contrast Bouhanni, who had space to his left to swerve into when Demare cut across him.)
That was my initial reaction when watching it live, and none of the slow motion replays or alternative camera angles have made me change my mind.
I think the DQ is harsh, but I'm glad it wasn't my decision to make - bit of a no-win situation for the jury with so many pundits, DS and riders having their say, all with conflicting opinions.
The cynic in me would like to point out that two significant entrants in the green jersey contest are out of the way and a Frenchman got a stage win and lots of points
The realist in me says that Cav was never a contender for the green jersey in this year's race.