I'd like to chime in here with some questions as well, along these very same lines, I've had my 5DMKII since 2007 and didn't upgrade to the 5DMKIII, but now the 5DMKIV looks like something I'd take more advantage of than I am my 5DMKII right now. I've been looking around for a camera to do some VLOGs with and just a better over all video camera than the 5DMKII has feature wise. The autofocus would be a big deal to me, not being a video pro, I'd love it to just carry around and snap some video of the kids and grab everyday video type shots for a VLOG.
Anyway the key would be to sell my 5DMKII and 3 lenses I have, maybe only 2 of them, and a lot of the accessories that are made for the 5DMKII only and try to fund the new camera and lens purchase. I'm sure a lot of folks have sort of done this, maybe not their lenses, but I feel its time to upgrade lenses, I have the 24-70 2.8 MKI and 70-200 IS MKI and while I love them and they have done great, I think I want to go prime on most of my new lenses and get one VLOG type lens such as the newer 16-35 MKIII. I know I'm not going to get as much out of these two lenses and my old body either but I don't have enough cash to just go balls to the wall all out of my pocket expenses, it would be nice to have a little bit from the sales.
Does this sound like a reasonable way to upgrade and based on the changes from MKII to MKIV is it worth considering? I think its time, though the MKII is a great photography camera, I know the MKIV would be as well but also serve the VLOG and video needs I am wanting to start up. The improvements now, seem to tell me its time and worth this effort. Just want to make sure I'm not crazy?