Chris Slade has said they were going to get either someone from an AC/DC tribute band or just a random singer that no one has heard of. Personally I would have loved to see Jimmy Barnes take the role if Axl didn't. So from that perspective, I'm glad they picked Axl. It got him back in the spotlight just as much as the reunion did.
With GNR, hes been in cruise control for over nearly two decades with endless touring and singing mostly the same songs. AC/DC is a much tighter ship. They don't do late starts or have constant sound issues, everything runs smoothly. Its a once in a lifetime opportunity to fill in as the frontman of one of the biggest bands of all time so of course hes going to give it all and then some.