I prefer not to use the 24-70/2.8, I use it THAT rarely I didn't bother upgrading any of my zooms. I didn't use it at all yesterday.
And any reviews I have seen commented whilst the adaptors work, they are not as quick as the native lenses on native cameras and I NEED every millisecond that I can get.
We are talking about very small percentages here, most people will all it a non-issue, but they are an issue for me, deal breaking issue.
p.s. I thought about getting the Sigma 35 Art too to replace the 35L, we'll see. I have the 50 Art too and it is nice.
But again, lack of thumb stick, lack of Dual cards = not in a million years.