Then explain the 777F, 757PF and 747F 😉

All sold or about to be sold alongside the booming PAX versions

But those where not available until the market had dropped a bit. First few years you will only see pax versions as Boeing is trying to sell as many planes as they can. Annd with the limited production slots available pax airlines are more lucrative.

Once there is a dip in the market the freighter is introduced. This may not mean that there are no more pax planes sold. It just means that less are sold then their production capacity. After all, when a plane is brand new it sells more then when it is, say, 5 or 10 years old. However, but that time the complete production infrastructure is already in place. Boeing would be stupid not to fill it.

The 777F does not exist. As far as I know Boeing is studying it, but that’s it. The 757PF was introduced at the end of the 757s life. Only the 747 was different. And that’s because it was designed from the get go to be a cargo plane. Back then it was common knowledge that SST’s would be ferrying passengers and big 747s freight. In the end the fuel crisis stopped that but that’s for historians.

Besides, the cargo airilnes where desperate to get the 747F and willing to pay for it. They will not do that for the 787 because there are loads of 2nd hand 767s and 330s available. They are cheaper for cargo airlines.