I presently have a 2013 MacPro with 64 GB.
Are you currently experiencing limitations with that platform? The reason I ask is that I have what I presume is an identical or near-identical machine (6-core 3.5GHz processor, 64GB memory, 1TB flash filestore, AMD D500 GPUs) and I still am quite happy with the performance of Lightroom and Photoshop. I back up everything across a wired gigabit network, which, if anything, is probably slower than your RAID. The design may be more than three years old, but I wouldn't be ready to invest in a replacement if Apple introduced it today.
Having said that, although I have no inside information, I would be surprised if Apple abandoned the Mac Pro product. It may not be a major contributor to the company's bottom line, but I can't imagine Apple's management wanting to give up the cachet of being an important player in the professional graphics market. Plus—and I don't think this is insignificant—internally, Apple's software engineering staff needs a high-performance machine to run the development environment and tools for its operating system and application products. I can't claim any special insight as to when a new product will appear, but I don't see the Mac Pro going away.