IMO the main problem is the vocals. People were already becoming very weary of her whispering on the Heartbreaker verses, but the chorus was at least full-voiced and super catchy. The Loverboy vocals were the last straw- I remember people saying she sounded like a dying cat.
I honestly think the song, as is, with full-voiced verses and chorus, would have done miles better and be remembered fondly.
(The video was just the final nail in the coffin)
PS.... I still love the song and video!!! (/still think All My Life as the first single would have changed everything about the era)
Failiing her being able to use the original Loverboy I think All My Life would have been a great start for her... wouldn't have been that pop success out the gate but may have eventually crossed over. But it would have really signified that it was Glitter was period piece and something a bit different for her.
Also, I live for All My Life and think it's amazing that Mariah worked with Rick James.
''All My Life'' is the better song, sure, but its commercial appeal was even worse then LB imo.
The good thing about releasing AML instead as the first single would be, as you said it, to emphasize that Glitter was a soundtrack from a movie set in the 70's.
But let's face it, Mariah did Glitter more as an album than a soundtrack, it is ALL her!