Whether or not Matthews actually follows through with his threat is still to be seen, and of course, that’s assuming the NFL passes a rule saying players must stand for the anthem, which may or may not happen.
As a private league and business, that is, of course, the NFL’s prerogative. It should be worth noting that the players are peacefully protesting, though, and they aren’t protesting the national anthem itself. This is a movement that started as a protest against the prejudice and oppression many minorities in America face especially as it relates to police brutality. It was a stance started by Colin Kaepernick, but the protests became a national debate and an NFL-wide phenomenon when president Donald Trump called protesting players sons of bitches.
If the NFL passes the rule and players actually leave the league as a result, national attention good and bad will only continue to shine on an issue that the league is desperately trying to move away from.
They say any publicity is good publicity, but for the NFL, only football publicity is good publicity, and this issue has gone far beyond the actual Xs and Os.