You can still make a profit if the belly is full of freight.
No you cannot. The premium-paying freight with guaranteed shipment times goes on dedicated freighters. Longhaul generic belly cargo which is loaded on pax flights is usually low yield, US$2.5-3 per kg depending on direction/destination. You need about 150-200 kg of freight to match average passenger revenues, you need a 74F full on both decks to make money on that. Rule #1 of passenger route economics: Never rely on cargo to make up for that difference to break even - it never will. If passenger loads make money, freight is a nice direct add-on to the bottom line.