The key thing is that assets of a 501c3 are irrevocably dedicated to charitable purposes, which is not the case for a 501c6. If the NFL had somehow started as a

The key thing is that assets of a 501c3 are irrevocably dedicated to charitable purposes, which is not the case for a 501c6. If the NFL had somehow started as a 501c3 before realizing that was a bad structure for their work, the standard approach would be for the 501c3 to sell their assets at fair market value to new for-profit. Then either those proceeds could be donated to other 501c3s, or the old NFL could continue as a foundation, applying those assets charitably. (Not a lawyer, just someone interested in non-profits. I'm on the board of two, but that doesn't make me an expert in regulation!)