There is only so much air cargo capacity, and passenger aircraft cannot be rapidly converted into that capacity. The cost is excessive to convert passenger aircraft to cargo service.
(am not an air cargo freight expert if you haden't realised already, so be gentle)
Obviously, you won't want to do this for general cargo transport, but for "hey, we have a grounded plane, but transporting air mail gives great rates right now" situations?
Limited loads could be secured in the passenger cabin using the same tracks. Whether or not that actually makes economic sense is a different discussion.
You can see this in action here: https://www.reddit.com/r/aviation/comments/8scjkk/ever_wonde...
Without passengers, passenger aircraft probably run out of volume capacity long before they approach their max take off weights.
Whereas I think with dedicated cargo aircraft, it is usually the other way around - weight is the main limit, less volume.
You'd allocate the volume in the aircraft very differently I imagine for cargo than passengers. I also suspect passenger planes were already using any leftover cargo from passenger cargo storage for commercial cargo transport to begin with. Two potentially very different optimization problems.
Now there are less passenger flights, so less capacity there! And less passengers to make the flight break even with people + cargo paying.
I think you are drastically underestimating the typical revenue of a long haul or international flight.
A good starting point is RASM, or average revenue per available seat mile. For the airline industry a reasonable benchmark for that is about 15 cents, or so.
So multiply the number of seats on the plane by the number of miles and you'll get something that's at least in the same order of magnitude for passenger revenue on a flight.
Since we know air freight only really makes sense for long distances or over water -- you're going to be using a truck to get medical supplies from Dallas to NYC, and it will take less than 24 hours -- let's pick a good example route:
HKG > LAX on a 767, a standard wide-body
7,243 miles x 375 seats x $0.15 = $406,912.50
So we know the post guessing $30k in passenger revenue for a typical flight is in the wrong order of magnitude.
For the number I kind of picked 15 cents out of my ass, it's a number I've seen in the past year in earnings reports for the majors and it easily gets us into the basic range we're talking about. Just google the terms RASM, PRASM, and CASM and you'll find a ton of info, that's the basic jargon used in the business to come up with info of this type.