I don't give a screech who joins Angus on stage when he does his thing, it's all yawny after two or three songs to me.
However, it's no less credible than a solely Francis Rossi-led Status Quo or a "Rainbow" with Blackmore being the sole member who ever played on a Rainbow album. To the non-muso public Angus Young personifies AC/DC and Axl is at least someone they have heard of before.
People don't go to Bob Dylan concerts because of the particular bass player he plays with either.
Rodgers in Queen was an awful choice. Not because he wasn't Freddie (most people aren't), but because he was on opposite ends of Freddie, yet had to sing his songs. Rodgers is a great vocalist, but one thing he cannot: Sing tongue in cheek, yet with operatic grandeur, very much English Music Hall. And that is key with Queen. Rodgers patterned his vocal style after US-American blues and soul greats. Lambert is doing a far better job than Rodgers (who never sang a wrong note with Queen) - as Roger Taylor once quipped: Being gay helps the role!