$640,000 might be the amount lost putting on the "next" game, but it's not the average loss per game. Passan's calculation of average salary per game less average local TV revenue per game misses out on all the national TV money, as well as any non-gate, non-tv revenue earned by teams (if you thought the ads on the walls around the field were bad now, just imagine what we're about to see). Those two numbers totaled somewhere around $3.6B ($10B in total revenue, less 40% of that coming from the gate ($4B), and another $2.4B in local tv rights) last year, or $1.5M per game. Considering that we're likely to get more national games than usual, through more games of the week and an expanded playoff, and every inch of wall covered by ads, and then virtual ads like we see at soccer games, I would bet that the majority of that $3.6B still comes in.

some sympathetic to the union acknowledge the league’s finances have been hampered.

Well of course it ####### has. The question is how seriously. We'll never know without the open books, whereas we know exactly what the players lose.

and every inch of wall covered by ads,

You sure about this? Are companies locked into advertising contracts? (I genuinely don't know the answer to this.)

If there's a Phase 2 pandemic, would companies be in a position/have a desire to blow a lot of money on advertising?

This is one of the best articles I've read on the subject -- using reasonable estimates and calculating the difference between different courses of action. You would really hope that a difference of a few hundred million in total salary could be overcome by motivated negotiators.