8 or 9,000 crowds at the Lane weren't really the norm - there were quite a few in the third division, and even one or two as low as 7000 in the second under McEwan. They were more common in the mid-80s when football generally was in dire straights thanks to decrepit, rotting stadia and decrepit, rotting brain-cells of a lot of "fans". United were on their knees financially and couldn't do anything to bring people in. Even Liverpool were struggling to get more than 30,000 a game despite being European champions. Arsenal often had gates of less than 20,000 and averaged around 24-25,000 in the mid 80s too. Chelsea's were relatively poor even at a similar or lower figure up to the mid-90s. Wolves had a spell in the early 80s of regularly getting 6000 or less in the top division, Birmingham and Middlesbrough were clubs that had top flight crowds of less than 5000 on multiple occasions, and of course Wimbledon would struggle to get more than 3500 if it weren't for travelling support.
And though they'll never admit it (they've changed their own record books now), Glasgow Rangers got a recorded crowd of 2,000 for a league game against Greenock Morton in the 81-82 season and that was supposedly an over-estimate - only a fraction paid on the day and no-one could be bothered to tot up properly (and they had many more attendances only marginally higher than that). Think those are recorded in the Rothman's year book of the time.
Football's changed now since Italia 90, EPL/Sky, sanitisation, Euro-96, all-seaters/new stadia, etc and clubs like United have got better at building and maintaining their support levels... but it still requires a team with an expected chance of relative success to keep the punters coming in.