I think the move makes sense in a way. I'm a 5th gen 4Runner owner and have considered switching to a GX460 in the past. In the end decided against as it didn't make sense financially since my 4R is already pretty built up and I didn't want to want to start a build all over again. I'm pretty happy with my 4R, there are things it does well and others that it doesn't do so well and while the luxury and the V8 of a GX460 were nice it wasn't enough to justify switching vehicles. Now that I've had about 1-2 years in researching and browsing the forums and GXOR here are a few things I've observed from GX owners. There's a bit of a superiority complex and almost a disdain for 4Runners, especially 5th gens. This could be for a couple of reasons. The popularity of 5th gens, the number of frat bro's and newb's that simply buy them because of an image, and yes I'll agree there are many things that make the GX a much better vehicle and value than a 4R.
GX owners identify the platform more as a Land Cruiser than a 4Runner even though we all know that the the GX comes off the same plant and factory line as a 4Runner. I think there's already a forum thread about that on here somewhere and many GX owners said it was more like a Land Cruiser than 4R. The only thing the GX has in common with a Land Cruiser is the nameplate internationally as as Land Cruiser Prado but you'd be surprised at how many GX owners like to point that out. I'm not trying to incite a war between the platforms but more or less my observations as someone who owns a 4R and has considered the GX and still consider it under the right conditions. I think GX owners would love nothing more than to distance their vehicles from the 4Runner. So in a way separating the forums to me makes some sense. I also think there are some inherent features and challenges that both the GX470 and GX460 have in common that the 4R's don't. There are still problems finding good bumper options and poorly retrofitted Australian ones haven't always been a direct fit given the unique body lines. Then there's the shared barn door and some versions having the air suspension and height control. Again, features and issues that the 4R don't necessarily face.
I mostly hang on t4r and I can see if that if mud wants more 4Runner presence on here they'd create a dedicated subforum for it. Also with the rising popularity of the GX platform (470 and 460) it's probably easier to see like and similar builds, mods, and problems for that platform than to group it with the 4R.
While I can somewhat agree with what you're claiming here (very small percentage), I have to say that the complex you're pointing out is few and far between. It's overwhelming how many of us GX owners are former 4Runner and Tacoma owners, it would make very little sense to bash on and look down upon the very vehicles that many of us once owned and loved. There are quite a few owners who converted from older series Land Cruisers as well, in theory that would be a step down to most but it's all just part of growing and adapting to your needs at certain chapters in your life. Both the GX and 4Runner have their pros and cons but I think most would agree that the GX is a nicer and more well rounded vehicle, that doesn't mean it's necessarily better.
The biggest issue we are complaining about here is the lack of separation amongst the generations/series of the GX model (which appears to be coming soon). The 120 series and 150 series share very little other than a namesake. While the design aesthetically is similar due to an evolutionary change, the underpinnings of the vehicles are different. I think it's great that the 4Runner and GX will now be separate and no longer part of the same series subform and it should help new comers to more easily navigate the subforms and it might even dispel some of the snobbery you mentioned since the owners will no longer be amongst one another. I doubt that this change will redirect much traffic from T4R though, that forum is a monster in comparison.