The 5D4 was my primary SLR. Since I got my RP and then the R, I have rarely used my 5D4, 7D2, or 80D. I plan on getting the R6 once it becomes available refurbished. The R5 is overkill for me. It has video capabilities beyond what I need, 45MP is more resolution than I need, and despite cheap storage, the files are bigger than I want to deal with. Plus the R5 costs more. Why pay more for specs and functions I won't take advantage of? Do I wish the R6 had 30MP instead of 20? Yes, but otherwise it offers all the performance I need. By offering the R6, Canon will be getting some of my cash. If only the R5 was available, I wouldn't be updating anytime soon. If the R6 had lower resolution but better low-light performance as you proposed, I wouldn't consider it. Resolution below 20MP is too much of a sacrifice for high ISO performance for me.
My only issues with the R are its relatively slow burst speed and the lack of IBIS. The R6 fixes those things without giving me huge resolution and pro video features I don't need, and it saves me money compared to the R5. There is definitely a place for the R6 in the market.