Really? I recall Byrne issues running 600K copies a month, with return rates under 40%. That's a successful book.
I don't know the figures you are talking about, but I do know I have read John Byrne state multiple times that X-Men was not a huge success while he was on it. I have read other creators say the same thing. Sales were growing, and the book had become successful enough that the book went from bimonthly to monthly, but it was not the huge hit it became in the 1980s and would not have lifted the company by itself.
This link has some information on the improtance of the Star Wars book and the print runs of the other comics.
http://io9.com/5840578/how-star-wars-saved-the-comic-book-industry
It specifically states that Spider-Man was the best selling book and sold only 280,000 out of a print run of 590,000. Your print run of 600k and 40% return could very well be the case by the time Byrne's run ends, but it would place along those same lines. A popular book, but not something very different than its other top selling titles.
On the other hand, Star Wars sold a million copies off the bat. That's not printed, that is SOLD. And it would have happened at a time when X-Men probably sold between 100,000 to 200,000 sold.
So in the end, the Wein/Claremont/Cockrum/Byrne run did not have the same impact as Star Wars did, and it did not substantially change the financial condition of the company. Eventually with Shooter coming on board Marvel recovered, sales increased across the board, and Claremont's X-Men became very important to their success. That was not the case in the mid-seventies though.