I guess it depends on your perspective of what "soccer becoming a big thing" actually means. 30 years ago, the US didn't have a real professional soccer league. A generation later, and you've got MLS averaging over 23.5k per game across 29 teams. Ratings for the Copa America have been very strong, the last world cup was very strong, and I'm sure the 2026 World Cup will blow away all prior record ratings. If people were expecting everyone to suddenly stop caring about other sports with soccer being king, they had completely unrealistic expectations. But it continues to grow year over year, generation over generation.
Now that league did die out in the late 1980s after operating for about twenty years, but when you look at the numbers you give and compare them to then you just do not see the growth you might think.
Fortunately I think the current league does have more Americans playing on each team than the old league had playing back in the day.
Granted, I was just a kid at the time of the first version of the NASL, but were there any other pro leagues in the states at the time? I don't believe that the support structure for the NASL was nearly as deep as what are in the States right now.
Currently there are five men's pro soccer leagues in the US.
Level 1
MLS with 29 clubs
II
Level 2
USL Championship with 24 clubs
III
MLS Next Pro with 29 clubs
NISA with 9 clubs
USL League One with 12 clubs
On the women's side, the NWSL is in their eleventh season with 14 teams. The USL is supposed to be starting a second pro league starting in August with 8 teams.
I believe that all of the MLS & USL teams have fairly sizable academy programs set up. Hopefully their finances allow to catch talented kids without the financial means to pay for the typical pay for play clubs.