I too was Bon Scott/Brian Johnson. It took me ages to get used to Brian - and I still think Bon was the best.
I have not really kept up with who is who so can't say quite who I would choose.
Certainly, vocalists seem largely to decline as they age. Early David Coverdale in Whitesnake was truly a talented singer - later, not so much. Likewise Ian Gillan in Deep Purple, who I saw at the Hammersmith Odeon many moons ago. Awesome..later, not so much.
Maybe Rick Halford from Judas Priest could do AC/DC but I have not heard him sing in yonks so do not know if he could still hack it.
Rick never sang for Judas Priest.. Rob on the other hand..
Told you I was old.....memory...errr...you know - thingy - ummm
;) as long as its not Rick Astley.. somehow Painkiller would not be so .. intense..