One, OP's question was about the threat model of underground-gym-goers not the CDC and state/local medical authorities.
Two, public health authorities don't do "exactly that calculation" because they are looking at interventions on the population level. Their job is to mandate things that would be individually irrational for the collective good. They have their own calculus about what the acceptable trade-offs are, that I'd suspect ultimately relates to the same monetary value of a human life that FEMA uses, but I'm not sure. I generally trust them to do their job correctly though.
One, OP's question was about the threat model of underground-gym-goers not the CDC and state/local medical authorities.
Two, public health authorities don't do "exactly that calculation" because they are looking at interventions on the population level. Their job is to mandate things that would be individually irrational for the collective good. They have their own calculus about what the acceptable trade-offs are, that I'd suspect ultimately relates to the same monetary value of a human life that FEMA uses, but I'm not sure. I generally trust them to do their job correctly though.