BelArg :
Always good to discuss. But those examples you give are of pairings which in the general scale of things are thin traffic, and almost entirely visiting-friends-relations, down at the bottom of the revenue curve. This is nothing more than fill-in traffic which readily diverts to an intermediate stop if they can save £50. Not the sort of revenue to justify a new type.
Conviasa of Venezuela now do a route Caracas-Damascus-Tehran, one of the examples you give. A recent enthusiast's account on the web described a trip on it. 24 (!) passengers, on an A340.
What justifies investment is traffic flows which balance to the fleet's typical mix of F/C/Y seats, which are not highly seasonal, and which have a good amount of business, government, and other premium-class traffic, good opportunities for connecting passengers at both ends of the route, and freight for the belly in both directions as well.