Both these cars could be the same ones, even if produced in different regions, and also have different names, something that is not unusual at Toyota.
I do not believe the NA versions (Tacoma/4Runner) are significantly less durable than versions (Hilux/Fortuner), or that it would cost too much to make the Hilux/Fortuner NA viable. Toyota's BOF lineup is from a time the market was not SUV/CUV crazy, now it also has to adapt. The main issue seems to come for the development and life-cycle. the BOF car Sequoia and Land Cruiser are really very old. But Toyota is not as small as Mitsubishi with their old, yet very well built and still competitive Pajero.
At this point I think we should move this part of the discussion to the BOF thread:
https://lexusenthusiast.com/forums/threads/lets-talk-bof.4076/
Your assumption that Tacoma and Hilux can merge into one platform is not true. In the U.S., majority of the road freight is transported by semis, and pick-up trucks are mostly for hauling divisible cargo by individuals and small businesses. In fact a lot of people use them for recreational use. Also, crew comfort and safety are very important for american buyers. In the rest of the world, pick-up trucks are mostly used for commercial cargo transport, and safety regulations aren't as strict. Light trucks are much more maneuverable than semis in tight cities of Asia and muddy roads of Africa. Due to the scarcity of heavy-duty transport, pick-up trucks are overloaded on a regular basis.
As a result:
NA-spec pick-up trucks are much more front-heavy because of extra crew protection, space, and amenities. The payload capacity is heavily underrated.
RotW pick-up trucks have less weight in the front, but much higher payload capacity for the same size and power.
The Tacoma is a slightly larger vehicle than the Hilux. But the Tacoma is only rated for 1600 lbs of payload while the Hilux has 2,000-2,500 without any modifications. To get that kind of payload you would have to go for a F-150 with the payload package that strengthens the rear axle, but that's a much bigger and more powerful truck than the Hilux. Of course the Hilux gets a very torquey turbo-diesel (seriously, americans deserve these diesels!) that helps a lot, but most of that impressive capacity comes from a very strong rear, which is not practical for the U.S.
When designing pick-up trucks, engineers always tune the chassis to have optimal drivability when the truck is
fully loaded. Doing so requires the rear suspension to be much firmer than the front so the ride frequency could match. The brake bias needs to be towards the rear so the truck won't nosedive and lose traction under heavy braking (which is why Tacoma's puny rear drum brakes are a big problem). But such a set-up is both terribly uncomfortable and unsafe. When the truck is empty, it will have a tendency to oversteer and roll-over (remember the infamous Hilux moose test?) because the front wheels have too much grip compared to the rear. Of course the issue could be solved by stability control, but it should only be used in an emergency and there is no replacement for a stable chassis. Ford paid dearly for the notorious Expedition roll-overs. As a result, american manufacturers nowadays are willing to sacrifice capacity for extra safety and comfort. That's why they build the trucks to be heavier in the front and less stiff in the rear.
My point is there is room for people that want a real old school image BOF like the G-Wagon and LX and for those that want a new school image vehicle like a car based suv flagship. I know many G63 owners (more than the 550) and they don't give a crap about its off road capability. To them its cool and hip and makes them stand out from the crowd since its not that popular (since its so expensive for one lol). The LX kind of has that same flavor here, old school, hip, stands out from the crowd in the USA. I don't mean to say the G63 and LX are head on competitors exactly. 
