To put things into perspective: Using auto iso 100-800 on crop seems fine to me,
I found that to be the case for the 40D, but things started sliding downhill with the 50D and 7D, IMHO.
the sharpness of an image is more dependent on the lens (think ff+cheap lens or crop+expensive lens, not $1000 vs. $5000),
Sensor quality plays a huge role in sharpness. When using my lenses on 5D III, 1DX, and 6D, it is like I'm using all new gear with better color and sharpness.
Last not least if people spend a hilarious amount of money on a gadget I'd wager to say it's tempting to rationalize a fun purchase (and the 6d has great iq) as essential even when in many situations crop would deliver the same result for standard print/view sizes.
There's no question that a user can produce outstanding images with crop. But that same user will produce even better images with FF. The images just simply "pop" more due to being cleaner, sharper, and more colorful.
I'm also not a big fan of the sell & buy game and rather stick to what I have and purchase other things that are also important (esp. lighting gear (flashes, diffusers), but also monitor, color calibration, tripod, filters, printer, software ... repairs!). For crop your 11-16, 17-55, 70-200 should indeed about cover it, the 17-40 & 24-105 are really ff lenses in sharpness & zoom range even if they add weather sealing.
I agree about the buy and sell game. People get addicted to it. *But* there is validity to it in certain cases, and going from crop to FF is one of those.