The 50L doesn't get any worse now that the Sigma is coming out
and for many that have the 50L, the decision is not as clear cut, especially if they're waiting for the 50L II to come out before making a decision to switch the S50A.
So what do we know about the "mk II" of any Canon lens? First, it is more expensive than the "mk I" and by some margin too. So that would put a 50/1.2L Mk II at close to (if not more than) a MSRP of $2000 at launch - twice the price of the Sigma 50/1.4 Art. Next, unless Canon have also moved away from using a double gauss design, their lens will be subject to many of the same flaws as the current one. Now if Canon were to use a similar design to that of Sigma and Zeiss, it could be better but in order to be significantly (or even noticeably better) than the Sigma lens, it will have to equal or exceed Zeiss's Otus. I just can't see that happening. I'm not saying that it won't happen, but Zeiss lenses are considered the pinnacle of lens design for good reason. One thing that Canon can add to justify themselves is image stabilisation.
Sigma is asking some serious questions of Canon and also Zeiss.
When do you realistically expect Canon to deliver in a 50/1.2L mk II?
The 28/2.8 was 25 years old when it was replaced by the 28/2.8 IS USM.
The 35/2 was 22 years old when it was replaced by the 35/2 IS USM.
It was 18 years from 85/1.2L to 85/1.2L II.
The 35/1.4L is now 16 years old (without a replacement).
The 50/1.2L is only 8 years old (without a replacement).
How soon do you realistically think it will be before a 50/1.2L mk II appears?
With Sigma's success with the S35, I'd expect the S50 to perform well AF-wise. Over time, I'd suspect that many would trade the 50L for the S50A, but it is a bit premature when it still is not available to everyone.
And if one has the 24-70 II, then the 50L would be used for portraiture/low light only, so edge/corner performance is not as important. The S50 is even better than the 24-70 II at 50mm. Is everyone now going to say that the 24-70 II sucks too for landscape and that the S50 A should be used instead?
Let me answer that by saying that if I had both the 24-70 II and S50A in my bag and whilst using the 24-70 to frame a particular shot I found that it worked at or close to a zoom of 50mm then I'd swap the 24-70 II off and put the S50A on in a heartbeat (unless I had no time.) I actually do this quite often: if I'm using the 24-105 for landscape and a particular shot turns out to be a 35, 50 or 85 then I'll swap the lens off for a prime without any qualms.