Again the NFL already has a contingency plan in place that involves reducing the schedule if they have to. Multiple owners have gone on record on this, we want to play a full season, if it gets to a point where we need to shorten the season in order to complete the season, that's on the table. They THINK that won't happen but they don't know.
The estimate from CBS is that each home game without fans COSTS a franchise 10 million so losing a full season is 90 to 100 million dollars(the 2 preseason games too). Their plan is not to play the whole season without fans their plan is to get to where they can play with as much as 50% of the fans, that of course would require a working vaccine.
As the saying goes, everyone has a plan until they get punched in the face, the NFL hasn't been punched in the face yet. I hope it works, so far it's been very good, but going 4 months without issues is going to be a concern because they can't do the bubble like the NBA did. They don't control what the rest of us do, if you look it appears we're going to be spiking up in cases in the next few months based on OUR behavior, opening up schools, events like Sturgis etc. Hopefully the NFL can keep personnel and players safe from all that but there's no guarantee they can.