Boston was last in their division that year. Of course their attendance would be down. That doesn't mean the league was going under. We went to Bulls games and sat where ever we wanted until MJ's second season in the league - 1985-86.
I already stated that the ABA diluted the talent pool and splintered the professional basketball fans into 2 camps. The ABA operated from 67 thru 76. That was the period that weakened the strength of the NBA product. But the leagues merged in 76 and NBA basketball quickly became a powerhouse in a few short years.
Before Magic joined the Lakers, they had Kareem, Norm Nixon, Michael Cooper, Jamal Wilkes (remember Cornbread, Earl and Me with a young Larry Fishburne) and Adrian Dantley. Showtime was already in place.
Dr J, George Gervin, David Thompson, Kareem, Darryl Dawkins, Artis Gilmore, Moses Malone, Rick Barry, Walt Fraizer, Bill Walton, Elvin Hayes and Willis Reed were all in the league during the 70's. Of course the league had star power. Moreover, the league hadn't morphed into a star driven league yet. It was team oriented and it worked.