I think you're right about people responding to Bayformers because they represent characters from childhood, but I would add in all of the different verses, not just the old books and animated series. Also, I think participating in the fandom adds another layer to that, filling in all the blank spaces in Bay's characterization. For instance, making Ironhide a close friend of Will and overprotective guardian of Annabelle, or writing fic that mentions Sideswipe missing his twin even though Sunstreaker was never even mentioned in the movies. (Did Sideswipe even have any lines? I can't remember.)

I think all of these things end up blending together, allowing fans of the movie to care about the Bayverse bots.