re: Styx, you have to remember they had basically no control over their first album. Lady was supposed to be on the first album but the label didn't approve of it and it was eventually put on the second album instead. According to DDY, the label hated that song and that's why it didn't become a hit until 3 years later when Equinox (the 5th album) came out. Meaning the band, themselves, really had what it took from the beginning, just not the backing.
Really, if you look at the number of songs that totally could have been hits toward the beginning like Best Thing, You Need Love, Rock and Roll Feeling, in addition to Lady, it's no surprise that once Styx had the freedom to make the albums they wanted to make with the proper backing, they easily had everything it took to make it right away.
Add to that the addition of Tommy Shaw for their 6th album, and we go from moderately successful underground 70s prog band, to chart topping stadium headlining supergroup. (Of course I like their music with JC just as much musically, though clearly it wasn't as commercial.)