So, just thinking about this a bit more, when the 5Ds/sR came out, the ISO was limited. I've always thought that it was limited as Canon's file size increases with increasing ISO. Using the R5 as the basis here at ISO 100, the 45 MP RAW files are 51.6 MP (using TDP review as reference), ISO 12,800 the file size is 66.4 MP, and ISO 102,400 the file size is 79 MP.
I suspect that FPS decreases with increasing ISO, but I have not seen that confirmed. But, assuming that the R5 can do 20 fps at ISO 12,800, then the throughput it can handle is 1,328 MB/sec. Playing with the math, if the 90 MB (I still suspect this is the 82.5 MP scaled up from the 90D/M6 II) file size is ~1.5x the resolution size in MP, that would be 135 MB per image at ISO 12,800. Divide 1,328/135 and we are looking at just under 10 fps.
If this had 10 fps with mechanical shutter and say the ability to go faster in crop mode maybe CRAW, limit it to ISO 12,800, and that is a heckuva camera. For those concerned about file size (which includes me, to an extent) CRAW is impressive.
I would have to do some mental gymnastics as to why I (personally) would need this many MP as I am very happy with the 30 MP on my 5DIV. But, still...I like options.