Flying wide bodies and being a pilot for 26+ years it's about $$$$ plain and simple. Four engines burn to much gas, more MX,more of everything that cost money. Cargo pays very well, we will bump pax for cargo if It pays (call it weight restrictions,,,) The A380 is dead it just has to fall over, special gates, runways and so on, no cargo variant. The 747 will hang on as a freighter till the big twins kill it too.
Ps as a whale driver it saddens me to say this.