By the time that Stained Class rolled around, the blues influence in Priest's music was negligible. The similarities between Bruce Dickinson era Maiden and Judas Priest circa that album are not even remotely coincidental, even though all of Iron Maiden's music places a much higher emphasis on catchy vocal melodies than early Judas Priest, which is probably also why most casual metal fans find them more accessible.
Judas Priest was definitely a better band in terms of originality, songwriting, musicianship and vocals even though Iron Maiden's golden era was far more radio-friendly and instantly accessible, even on albums like Seventh Son of a Seventh Son which are supposedly departures from the norm and considered very progressive by Iron Maiden's standards. Despite that, it isn't groundbreaking like Sad Wings of Destiny or Stained Class and neither were any of their other albums even though their early music showed a lot of potential to go in a very unique direction that was quickly abandoned in place of a much more commercial and already established path.
They didn't push any new boundaries, at all, and despite Bruce's showboating wildman image, his vocals were actually very tame by metal standards compared to his counterpart in Priest, much like the rest of the band compared to early Judas Priest. I don't see how blindly sticking to already established metal norms five or more years late qualifies you for reaching the apex of heavy metal or any other type of music. I think their main accomplishment was giving metal more commercial exposure than it previously had, which is due to the qualities of their music that I have already explained.
I do have to say that Paul Di'Anno was the most unique asset that the band had. I don't blame them for removing him based on his behavior hindering their success and musical direction, but I don't remember hearing any other metal singers similar to him. There were other singers like Bruce Dickinson, however, before and after his rise to stardom.
In summary, a band that chooses to stay within an already established archetype several years after it has become standard is not boundary-pushing and doesn't reach the apex of anything unless your expectations are very small.
