AC/DC has now lost 3 KEY members (2 of them original members) in the last year or two. I feel bad for them, but maybe Angus needs to just hang it up.
It's been a lot of change in a short span of time. Losing Malcolm was a huge loss to begin with, for obvious reason. Losing Phil Rudd wouldn't have been a huge deal alone, and they replaced him with a former established member, but together with Malcolm, that was a huge hit to the rhythm section of the band. And now on top of that, they've lost their frontman, the voice of the band for 36 years. That is arguably the most difficult spot to lose, because it's what people identify with. It's a make or break factor for many bands, and many times the difference maker between filling arenas, and playing pubs.
I'm still glad they managed to complete that last album (one of my favourites too), and am fine with them wanting to finish off the currently scheduled tour dates though.
Canceling the tour would have been the easiest and safest option, but obviously they didn't want to go that route. With that in mind, I think it was a good approach to choose an established frontman that was a household name in their own right. They're going to lose sales without Brian Johnson whoever they choose, but they'd lose more sales if they went with some no-name cover band singer, no matter how good. Brian Johnson hasn't been a great live voice in a long time, so I don't think that's the most important factor in choosing a replacement.
Of course the double edged sword of an established vocalist is that they come with their own baggage, bringing in new fans, and also pushing away others. Axl Rose is certainly a polarizing individual, and many people are put off by that, although personally I don't think it's a bad choice, and I think there's a decent amount of overlap in the Venn diagram of GNR / ACDC fans among the wider general public of rock fans that ACDC appeals and caters to. They weren't going to satisfy everyone regardless of who they chose.