Boeing have a clear view, this from their current freight demand forecast:
Global Freighter Fleet to Grow More Than 60% to 3,260 Over the Next Two Decades
A combination of 4.0% annual average traffic growth, measured in tonne-kilometers, and a proven need for dedicated freighter capacity means the freighter fleet will grow by more than 60% over the next two decades. By 2039, 2,430 freighters are forecast to be delivered, with approximately half replacing retiring airplanes and the remainder needed to meet projected traffic growth. More than one-third of these deliveries will be new widebody cargo airplanes; nearly two-thirds of the deliveries will be conversions from passenger airplanes.
source:
https://www.boeing.com/commercial/ma...argo-forecast/
FF