Until the Old Firm gates explosion, starting with Rangers in 1986, keeping home gates actually benefitted the likes of Aberdeen, Hearts, Dundee Utd, Hibs (and St. Mirren), along with having a smaller top division.
The differential in gates wasn't huge in the late 70s and early 80s and so these clubs were able to compete more equally with the OF.
Of course, it also helped that freedom of contract didn't exist and clubs could build sides over a number of years and keep their best players.
Saints kept their top players for a number of years and were able to sell them for big money. Indeed, back in 1979 St. Mirren were able to outbid Celtic on both the transfer fee and the wages to sign Frank McDougall.
Impossible now of course.