Main selling-points for mirrorless SLRs in my circle of photographer colleagues are:
1. Frame rate
2. Elimination of mirror-bounce blurring at slow shutter speeds
3. WYSIWYG viewfinder
Most of them are invested in Nikon or Canon lenses and are ready to throw thousands of dollars at the first mirrorless SLR from either of them which is exactly the same size as current mirror bodies. Many find the Sony mirror less bodies too small to hold.
I suspect that's the minority view. For most people (myself included), mirrorless (Fujifilm X-series) gives us most of capabilities of a DSLR in a far smaller/lighter package. But get above 135mm (35mm equivalent) focal length and those savings effectively go away.
Don't get me wrong, I use my Fujifilm camera more than my full-frame Canon DSLRs. But it's almost solely because the Fujifilm is smaller and lighter to travel with.
1,3 I rarely care about, and if 2 is a concern at such low shutter speeds, then break out the tripod, and exposure delay. I'll set mine for a second or two after mirror up, and voila. Plus, you're going to need new lenses, or extensions to adapt your existing FX lenses to it. The sensor distance won't be correct otherwise.
Of course, it will be fun to play with any new tech.
1. Frame rate
2. Elimination of mirror-bounce blurring at slow shutter speeds
3. WYSIWYG viewfinder
Most of them are invested in Nikon or Canon lenses and are ready to throw thousands of dollars at the first mirrorless SLR from either of them which is exactly the same size as current mirror bodies. Many find the Sony mirror less bodies too small to hold.