I think that's a stretch assuming the old Mac lost only 20% of its value after the new one is available. It's an unknown what the old one could be sold for, if the person was willing to sell it. Lots of people will give an old Mac away instead of trying to sell it, such as to a relative or friend. The only thing truly known is what the new one costs.
There are lots of reasons to purchase a 2019 Mac Pro over any of the current Apple Silicon Macs besides RAM. Graphics and storage options are also superior. For example, I have 5 additional SSDs installed in my Mac Pro. It would be a pain having a rats nest of external storage with a Studio or mini. It can also be put into a RAID setup for incredible speed.
Graphics performance of the top AMD GPUs is about twice that of M1 Ultra. You also don't take much of a hit with CPU performance. Multicore performance is about the same between the 28 core Xeon and M1 Ultra. So for many workloads, the Mac Pro will be faster due to much more RAM, faster storage, and faster graphics.