You dont really need more than 2GB RAM in RT, it's a huge burden. Not even more than a few megabyte. Also 64bit pointers are twice as slow as 32bit pointers. Just think of randomly jumping around in your terabyte ram. Filling the cache lines cost 50 cycles alone, and your budget is usually around 40-100. A 64bit rtos might be usable for image detection (self driving cars), but not much else. The cpu cannot be general purpose, gpu's can do that much better. I don't really see the market for such a thing.

It's not uncommon to have RT workloads where the "hard" RT part can fit in a small amount of tightly coupled memory, but the same processor is also used for "soft" RT workloads that need a much larger amount of memory. 64-bit addressing makes developing this kind of system a lot easier.