That's never going to happen. The NFL would play a full season with totally empty stadiums if they have too. They see fan attendance this year as a bonus, not a requirement.
At the best of times a slate of sold out home games accounts for about 25% of all team revenue (on average). I don't think any team expects to have more than 20% capacity all season. So at best fan revenue might account for 5% of this years revenue for some games. When you factor in 20% as the max later in the year and only at some stadiums it's likely this year fans will account for 2.5% tops.
They can easily miss that. In fact it probably costs them more to open, run and staff the stadiums and make them Covid secure than 10-20% capacity earns.
They make most of their money from TV. So cancelling games would just be losing them more TV money. If anything, I imagine their TV money will go up.
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.