Like I stated before, A-, A-maj7, A-7, A-6, is actually a really popular progression. Stairway is actually nothing more then a highly decorated version of that. Here's a couple things to consider.

- there's like 5 different sections to the song, every section begins on Aminor.

-Do you really think Page was thinking that when he wrote it. It's actually just an A minor tonality with a descending bass line and ascending melody line. If you do think of this as being in the key of C maj how do you explain the D/F# chord?