I don't really know how to approach the 'improve on the formula' type comments.. - while I understand them from a solely musical perspective, I can't highlight them in my own perception at ALL because I'm foremost concerned with the songwriting and if it moves me. If the band manages to do it and sound just like another band while doing it, or sounding like a totally different band, or sounding just slightly different, it makes no difference to me. Putting the former sort of comments at the forefront of an opinion makes it seem like some kind of freaking realism art piece is being analyzed in a museum somewhere, instead of music.
There's bands I like better than Priest and Maiden as a whole, and both Priest and Maiden have songs I like more than many songs by many bands. Their hype, their longevity, and their place in history make no difference to me, and I can't be influenced by such things if I'm going to attempt an honest MUSICALLY based opinion.
But to each their own.