> Out of curiosity, do they not pressurize the cargo hold?

Well, the DC-3 is a fun example, because it wasn't pressurized to start with.

But no, normally converted freight aircraft are fully pressurized; it's more expensive and more intrusive to redesign the plane to have a cockpit pressure bulkhead than it is to just leave the whole thing pressurized. There are some exceptions like the Beluga, usually due to door design constraints (at some point, making a cockpit pressure bulkhead becomes easier than making a giant pressurized door). This trend in retrofits might change; flat aft-pressure-bulkhead retrofits are becoming a thing to increase cubic footage capacity, and at some point someone might decide that the effort required to engineer and certify a cockpit bulkhead would be worth some advantage in door design or cargo capacity in a broader sense. But for now, they're usually fully pressurized.

The main reason why planes get a second life in freight is that freight carriers have _way_ more options for utilization; they fly fewer hours overall, hold the plane until it's completely full, and utilize different airports and routes. A loud, inefficient plane is OK to fly twice a day between two fixed airports with no noise restrictions, but useless to a passenger carrier who wants to make four or five turns between whatever airports are necessary and doesn't have guaranteed utilization to cover the overhead - right back to your original point, which I don't think anyone was really disagreeing with.