For what I do with my camera, FF can pretty much compensate for using a small prime instead of a huge L zoom, just by cropping the image. APS-C is a few levels below in this regard. FF + 40/2.8 can pretty much act like APS-C + 24-70 in term of usable image resolution for screen.
Using large whites is as much a reason for IQ than closeups. I use a 200 f/2 rather than the 70-200 f/2.8 for the better IQ. The 100-400L does not compare with the 400 f/2.8 either
Well, first of all, why would you use large whites with a mirrorless camera?
Cropping has the effect of reducing the bg blur that you would get with a longer lens
So, are you going closer to the lion just to get that bg blur you want?
(sorry, I couldn't resist)
Now seriously, if I put something like 85L on my 5D2, there is nothing in the APS-C world I could compare it to, even 85/1.8 on FF is unbeatable for the price (opposed to 50/1L on crop). FF wins in bokeh competition any day of the year. The distance is an important factor. To get the same framing with the same lens on both FF and crop cameras you have to be closer when using FF and that's where the bonus bokeh comes from. More on that, using a crop sensor camera and cropping FF image to match it (same lens on both cameras), gets you to a similar result without any compromises in bg blur. So, in my mind, FF is a win/win.
24-105L on FF >
17-55/2.8 on APS-C, while price and weight are the same + 24-105L has wider (15-65/2.5 equivalent) focal range as well as dust and moisture resistance. If not cropping, 40/2.8 on FF equals 25/1.8 on crop, which wouldn't be a pancake lens for $199 (or could it be?).