It's not even limited to RTS's. Try making a Baldur's Gate style game or a Ultima VII style game today (that is, a third person, party game with a rich environment) with the production values equivalent of Skyrim or Far Cry 3. It would be really really tough for the same reason - memory and cores (though mostly memory).
The 2GB limit was hit half a decade ago. Designers have had to work around this for years and it's slowly snuffed out certain genres. It's not as if there wouldn't be huge demand for a Baldur's Gate 3.
The beauty of first person is that you can greatly limit what is on screen. DirectX 10/11 lets you put your textures in a different address space so you can do amazing visuals in them.
Anyway, time will tell. We can revisit this in say 2 years and see where we're at. That's when I'd imagine 64-bit required games will start being the norm (I would be shocked if the next gen consoles aren't 64-bit).