This famed declaration of musical integrity appears at the end of the obligatory "thank you" list on Iron Maiden's 1983 album Piece of Mind.
Listening to the album (probably Maiden's best), it's clear that they really didn't have ulterior motives. Every second of music is testament to the band's uncompromising craftsmanship; where lesser musicians would throw in a few uninspired chords or perhaps a drum fill, Iron Maiden bridges the gaps with the intricate, dual guitar harmonies that put them head and shoulders above their contemporaries. And of course, there's not a synth to be found anywhere. Despite its commercial success, Piece of Mind was truly a work of creation, not a manufactured chunk of popular music.
I'm not inclined to speculate too deeply on whether the band subsequently developed any ulterior motives, but they seem to have run out of steam after 1984's Powerslave. Later Maiden albums are played and produced with expert proficiency, but the songs themselves seem mere rehashes of the band's earlier work. After five albums, they seemed to have run out of ideas. A telltale sign or not, the sixth Iron Maiden album, Somewhere in Time (1986) features ample doses of synthesizer playing.
Synths have their place in music. I have nothing against most of the modern varieties of electronic music, whose stylistic repertoire is based on synthesized sounds. But heavy metal loses a major bit of its primal vitality when toys begin to displace the unmistakable wailing and crunching of the electric guitar. The best performers in this genre are heirs to the old-school composers who created symphonies to fill entire halls with sound. And now a stack of amplifiers allows a few skilled musicians to generate power (both in the physical sense and the aesthetic) equaling that produced by a large orchestra. The "bigness" of the music is what makes metal special.
Did Maiden sell out? Not by any conventional definition of the term. They've never substantially changed their sound or image in all the years they've been playing. Iron Maiden riffs are still Iron Maiden riffs, even if they don't sound as good on a guitar synth as they do on a real guitar. Maybe that's the problem, though. Their sound, though certainly not designed to be commercial as the easily parodied hair metal bands of the 1980's or the contrived nu-metal currently in vogue, still brought them substantial mainstream success and a comfortable standard of living. Iron Maiden pays well, and the band has long since lost the will to innovate. Still driven to create new albums to satisfy recording contracts and maintain their lifestyle, they make slightly new songs out of slightly new riffs, just barely enough to avoid plagiarizing themselves. The synthesizers that first appeared on Somewhere in Time are not a symptom of selling out so much as a renunciation of the relentless perfectionism that made their earlier recordings masterpieces. By the time the band developed anything that could be construed as an ulterior motive, they did so out of complacency, rather than desire for greater fame or wealth.
Iron Maiden's individual musicians still had plenty of creativity left after the band's slow decline had begun, though. Bruce Dickinson's solo albums are powerful, highly individual expressions of what I suspect are a mere fraction of the singer's ideas that wouldn't work within the Iron Maiden framework. Listen to The Chemical Wedding a few times - recorded in 1998, fifteen years after the apex of the band that made Dickinson rich and famous, it's a genuine work of art without ulterior motives of any kind.