Part of the problem is there is no professional league in the US, so players have to go to Europe, South Africa, or Australia/New Zealand to play professionally. If there ever comes a day when American football is no longer played as its current version, flag football, or even with robots, then I don't see the networks suddenly filling that void with non-sports content. Most likely they will want to fill it with whatever the biggest fall sport.

Something else to consider is what happens at the HS level could influence what direction colleges and pros go in. A lot of HS play soccer in the Spring. Although some HS, especially ones that don't sponsor football, will play it in the Fall. Some will even do both seasons. If football goes away, then I am not sure you will see HS switch soccer to the Fall. I would think the more logical choice would be to keep soccer in its current season and final another sport to take football's place in the Fall. I believe just about all professional rugby leagues (league and union) are played during the Fall/Winter with respect to the hemisphere (northern vs. southern). So it wouldn't be a difficult transition for HS especially since a lot use the standard H goal posts which is similar to the goal posts used in rugby.