Two brilliant, high IQ cameras which are an absolute steal right now both on the used and gray markets. These cameras can run with low/mid tier MF backs in IQ while running circles around them in AF, speed, and responsiveness. They got pigeon holed as 'niche' cameras from day one which is unfortunate because they perform brilliantly in all scenarios including sports, action, wildlife, and yes, even low light. Aside from video...and these cameras were never meant to be cinema cameras...the only real complaints are 5 fps and the RAW buffer depth. Out of those two, I've personally only had an issue with the RAW buffer depth. 5 fps has been sufficient for me shooting surfing, volleyball, birding, airshows, etc.
The R5 will no doubt improve on base ISO DR, but will likely only improve slightly on high ISO. And it could be worse in terms of sharpness/detail depending on the AA filter. Here's hoping for a very weak or no AA filter on the R5. Either way, IMHO we're not going to see a substantial gain in IQ until we see Canon's next high resolution body. Anyone who can't afford the upcoming R5 but wants to compete when it comes to large, stunningly detailed prints simply has to pickup a used/gray 5Ds/sR.
I'm sad that there won't be a mark II version. I've said before that if Canon were to make a 5Ds mk II and a high resolution R with the same sensor/features, I would buy one of each. I still prefer OVF overall with a strong preference in certain situations. But I don't expect this given the state of the ILC market right now. No matter. My 5Ds isn't going any where. At current pricing it may even end up with a twin.