I saw that yesterday from the Guitar World FB page.
Very sad that Malcolm Young is in such rough shape, with no chance of coming back to the band. He's such a huge part of AC/DC, not just musically, but also in managing the band. And as you said, his rhythm playing was rock solid. He was only playing simple parts, but he mastered that role in a way that's quite underrated.
Black Ice was such a hugely successful album and tour for them, that I think it would have been a good time to retire. I doubt they need to keep touring at this point. That said, I'll still check out the album, but it won't quite be the same without Malcolm.