I don't think he was ever supposed to be a permanent replacement, he was just to do some shows with Queen as a guest lead vocalist.
However, during his run on American Idol, he got Robert Plant's (Led Zeplin) permission to perform "Whole Lotta Love," Slash invited him to try out to be the lead for Velvet Revolver and he also performed with the band Kiss in the finale, so in Queen's defense a lot of people thought he was good, or think he's still good now