Agree that it’s more about eyes on TVs than eyes on the field. The league was created for TV, not for attending fans. Loss of viewers and loss of TV revenue is what will end the league.
As for the NFL using it as a farm league, they already have the NCAA for that. And it’s a highly developed farm league that serves the NFL extremely cheaply and well.
If the NFL thought there was money in a spring pro league, they would have made it happen a long time ago.