I hear you, and that's normally good advice especially if the camera and lens were priced similarly. However, the 600mm prime is very likely to cost 2x that of the R1, which may put it forever in dream territory.
I already have the 100-500L, which is a very nice lens especially in good light. My photography life won't end if I'm unable to supplement it with a nice prime. Meanwhile, if the R1 performs as well or nearly as well as does the Sony A1, then that body will make my 100-500L a better lens while I continue to dream about the 600mm prime.