Your statement implies the point of FF mirrorless is to push spec barriers or to be small. Consider a different perspective.
Comparing to the 5D4...
If you don't shoot video:
- Your RF lenses have a control ring that you can customize, and you can make adjustments in 1/8th stop increments now
- All your EF lenses can be rear ND or CPL'd and they all get a control ring now as well
- Your mirrorless 5D4 now has Eye AF
- Your mirrorless 5D4 now has AF points all over the frame
- Your mirrorless 5D4 will (someday) be able to adapt non-Canon glass
- Your mirrorless 5D4 + adaptor ring now costs ~ $2500 (depending on which one you get)
- Your mirrorless 5D4 now has exclusive access to an industry first zoom and (possibly) Canon's first comprehensive workhorse 50 prime.
Additionally, if you do shoot video:
- Your mirrorless 5D4 now has a tilty-flippy
- Your mirrorless 5D4 can now output 10 bit 4:2:2
Yes, some stuff is taken away from the 5D4 experience, but so is the price.
Stop comparing to A7. Start comparing to Canon SLRs. This thing will sell very, very well.
- A