Video aside...I think the fact that the 5D2 offered a ~70% increase in resolution over the classic and prior to it's release you had to come up with $6k to touch 20+mp, I think it was very appealing for the portrait, studio, landscape and event still shooters and was a worthy upgrade from the classic.yes, but only for the tiny minority of cheapskate video folks who ofc were excited to get their hands on FF sensor and lenses at a fraction of the cost of proper video gear. For (majority of) stills shooters the 5D II had close to no appeal vs. original 5D.
Overall, the 5D II started the crazy wave of "absolute entitlement" and whining for "4k 24/30/60/120/240 (or 8k) in every single camera!" across all forums. And all camera makers succumbed to it. At the expense of the majority of customers who only need and would prefer to also have a choice of stills-optimized cameras.
The video feature really overshadows this now in hindsight, and seems like it was the only big deal about the camera. I think that is in part to the fact that 20mp is so pedestrian these days we tend to take it for granted but if you look for full-frame 20+mp options back in 2008 its easy to see why this was such a popular camera and it wasn't just because of the video.