Anyone that doesn't show up gets replaced by minor league players if necessary.
Start the minor league season immediately so you have a pool of replacements available. Sell 600 tickets or whatever the local community deems safe. I say 600 is a minimum because that was the average size crowd gathering at our local Wal-Mart during the peak of the pandemic.
While reasonable and logical, your proscription would result in a grievance filed by the players association that would be ruled on by an arbitrator and could result in the arbitrator enforcing the player contracts in full. That would cost the owners $Billions for nothing. They are better off not risking that and just use the limitations on gatherings above a certain size enacted by blue state governors to just cancel the season. That reduces their losses and more importantly reduces the potential of massive losses from an arbitration ruling going against them.