A lot of the cargo carried by air is time sensitive and valuable. It's not commonly large, heavy or bulky. Which means you may just as well pack the boxes on a pallet to lower-deck height, and load it onboard the ever expanding fleet of massive twin-engine aircraft, each easily capable of hauling 25 tons or more along with passengers and their bags.
Couple that with a seriously depressed general cargo market, which has been diminishing under the pressure above from integrators, and below from improved logistics planning allowing for slower modes of transport, e.g. road, rail or sea.
The result is rates dropping through the floor for general cargo, with plenty of evidence the general cargo market is hurting badly.
It might very well be, that in 5 or 10 years time the 744Fs are getting too long in the tooth and there's no viable replacement on the market. Trouble is, the carriers cannot fund today a need which may materialise sometime in the future.