They would lose because there is enough written in the league rules that the league could legally fine or suspend players for kneeling during the anthem.
"Paragraph 2 of the standard player contract requires that a player “conduct himself on and off the field with appropriate recognition of the fact that the success of professional football depends largely on public respect for and approval of those associated with the game.”
"a team might invoke Paragraphs 9 and 11 of the standard player contract. These paragraphs authorize the team to terminate the contract of a player whose “personal conduct [is] reasonably judged by club to adversely affect or reflect on club.”
"Article 42 of the CBA concerns “club discipline.” This portion of the CBA contains a clause for “conduct detrimental to club” and, under certain circumstances, it authorizes a team to impose a suspension of up to four games or a fine of up to four weeks pay."
Rule 5, Section 4, Article 8 "Throughout the period on game-day that a player is visible to the stadium and television audience (including in pregame warm-ups, in the bench area, and during postgame interviews in the locker room or on the field), players are prohibited from wearing, displaying, or otherwise conveying personal messages either in writing or illustration, unless such message has been approved in advance by the League office. Items to celebrate anniversaries or memorable events, or to honor or commemorate individuals, such as helmet decals, and arm bands and jersey patches on players’ uniforms, are prohibited unless approved in advance by the League office."