Well, my Sony a7r II is on order. The new camera has so many improvements over my current a7r that I’m expecting I may have trouble finding a buyer for my old camera. I’m still absorbing the advantages in the lens choice department.

• 18MP images with APS-C lenses. The a7r II can automatically switch to crop sensor mode when you mount a non-FE Sony E Mount lens. Thanks to the 42.5MP sensor, you still get images with 17.7MP of resolution using those lenses. I own a full set of the compact APS-C lenses that I used with my NEX-7, a6000 (and, soon, their successors), so won’t be losing much in terms of resolution or size.
• Minolta/A-Mount lenses at full resolution. I also own quite a few old Minolta A-Mount lenses, as well as Sony full-frame lenses that I used on my a99. I’m dying to test those out on the a7r II using the EA-LA4 adapter I already own. I have some A-mount APS-C lenses, as well, currently being used on my a77 II. They’ll work in crop mode at 17.7MP. It’s all good.
• Canon EF lenses with adapter. It’s likely that some people will switch to the a7r II from Canon, especially since it’s a few hundred dollars cheaper than the new EOS 5DS and 5DS R models, should match or exceed them in image quality, and — here’s the kicker — they don’t necessarily have to purchase all new lenses. Spend $650 for the Metabones Smart Adapter IV, and many (but not all) of your existing EF lenses will autofocus nearly as fast, and you don’t lose image stabilization or Av exposure mode.

Granted, using A-Mount lenses or Canon EF lenses on the a7r II defeats the advantage of having such a tiny, lightweight camera body. But I should note that not all of the E-mount and FE-mount lenses I’m currently using are all that petite, particularly my favorite, the 70-200mm f/4. Sony is adding native FE lenses all the time, but the ability to use a wide variety of optics (including Nikon and other mounts with adapters) opens up your options and, for many, eliminates or postpones the need to purchase special lenses for this great camera.