Fallacy. Tech stops are very expensive. Consider the additional fatigue cycles, engine cycles, time inefficiency, risk of going tech, etc. Fuel burnt during decent and climb are greater than the cost of tankering.
Older designs were unable to go non-stop with economic payload. Just because these older, less efficient frames are/were technically unable to do nonstop, doesn't therefore mean the Operator's desire is to 1-stop. Nonstop is better and the 77F was the first frame that can carry normal payloads sufficient distance to enable these type of network changes.
So the sales of the 77F were not greater because..........
Sales of the 77F have been great. Boeing has sold 250+ of them in 15 years which is impressive for a new build freighter.
For comparison only ~230 767Fs have been sold (to mostly 2 customers) across almost 30 years, and just 346 747Fs in ~45 years across all
variants (742F, 744F, 744ERF,748F). Not sure how many new build A300/A310s were made, can’t quickly find that info.
Further, the new builder freighter market was rather depressed over it's 15 year life span due to the rapid increase of passenger belly volume and weight capacity unlocked by the 77W, A346, A332/3 compared to the classical frames. It used to be that most freight was carried on dedicated freighters as the pax airplanes simply didn't have adequate payload remaining. That dynamic changed about the same time the 77F hit the market. In fact, a large number of air carriers dropped their freighter-only operations during this period and simply focused on belly freight.
So yes, as you stated, the 77F is the recent gold standard and sold extraordinarily well considering the challenging freight market. The 748F under performed expectations, largely due to the poor freight macro environment and heavily due to the success of the 77F.
The only other "modern" freighter to come out during that period is the A332F. It performed very poorly with a couple of operators even introducing it and then proceeded to abandon it (expensive mistake). One of the major problems with it is it's lack of range at economical payload (65+ T). It's not flexible enough in it's current iteration.
I still have doubts using 97klbs engines on an airframe significantly shorter than the -K. I don't think the v-stab is large enough for engine out performance at the elevated thrust level, and frankly, the h-stab may not be big enough for higher weights. It's much easier/cheaper to build a standard 900 into a F, take the small loss in payload, and make a stretch of it for integrators like DHL, UPS, FX that use lower densities. The plus side is this size sells on the pax front too as it better competes with the 78X (frankly, it should beat it on most missions).