But major cargo hubs like MEM and SDF do.
I don't know enough about SDF to comment on that, the majority of stands at MEM do not have direct terminal access, and I would think wherever the A300/MD11/777 park now would be able to accommodate an A330/A350. Again I think this is a red herring, they went from 108 ft on the 727 to 160+ ft for the 763F without the world stopping.
For a subject you claim to not know much about, you are making a lot of incorrect statements.
The 767F is not important for general cargo carriers. When talking about the 767F whether it be -300F or -400F the conversation really needs to begin and end with the integrators - FX, UPS, DHL, and really now Amazon. By "gate" this is talking about a parking spot, a silhouette, nothing to do with a terminal or jetway. They are clearly delineated at hubs and ramps and cannot be changed on a whim. At my ramp we have 5 gates - if you increase the size needed, you lose a gate. Which is not a feasible option.
FX got around the 777 wingspan increase by modifying the driving-under-wing policy to allow freight to move under the outer portions of the wing - which is never allowed on any of their other aircraft. Otherwise there is not enough space between wingtips to allow cargo movement. At MEM they also designated a few areas as 777 capable, spots where an MD-11F had a few extra feet to begin with, or spots where they used remote stands not up against the sort building making it easier to modify the gate lines.
The 767 is not using gates previously used for the 727 - the 727 was replaced by the 757 which did add about eight feet on each side but the original 727 gates had enough room to accomodate that. They could not do that with the 767 which is about another 15 feet on each side. Ramps on the other hand did have to make expensive changes to accomodate the new types, BIL for instance had to build an entirely new ramp on the other end of the airport that could accomodate two 757s at once, the old ramp could take two 727s but was just not big enough for the upgauge. It was a multi-million dollar move for a 2-gate narrowbody ramp. Now extrapolate that to a large widebody ramp with 4, 5 gates.
The 764....well, same wingspan as the MD-11 but much larger than 767. A350....same wingspan as the 777F. These are very, very clear ways to delineate the way the largest cargo carriers view these aircraft types. The A350 will certainly and absolutely not replace the 763F for the integrators. Again, I'm not talking about the Transaeros or Atlases or Kazakh Air Cargos of the world, their business model is completely different, they don't own dedicated gates worldwide and can park on any random hardstand. For the integrators, that is, the carriers who really drive worldwide air cargo movement, the projected 764F is the clear successor to the MD-11F, the 777XF is aimed at current 777F and 744F operators, the A350 is aimed at the same audience. The 763F is selling like hotcakes right now because the customers can see there is no real equivalent on the drawing boards in the near future, the A300 is aging and they've long ago made the decision not to operate the A330F. Airbus and Boeing are in a pissing match right now over the top end of the market, the 764F with new engines will be a move in the right direction and I think all it would take is pressure from an Amazon et al to really get the ball rolling faster on that.