Have we really gotten to the point where UHSII cards are too slow for our needs?
Yes. Exactly.
That is what happens when you spec 12 fps at 45 MP (rumored, but these are all rumors): 12 x 45 = 540 MB/sec. With Canon, as ISO increases so does MB/MP, usually up to ~1.5x. So 12 fps x 60 MB/frame = 720 MB/sec. At least in the EOS-R, UHS-II was only writing at a
maximum of 182 MB/sec. So, the math is pretty simple, yes, UHS II is too slow to support the specs of this camera. Thus, it gets down to buffer how long that 12 fps spec will hold if you are concerned about fps over a few seconds because after that buffer is filled and once you are writing directly to your UHS II card, you are looking at 3-4 fps.
I am not saying the R5 won't be a great camera. I am not saying the R5 will not be an improvement over the 5DIV. I am not saying the R5 won't be used for many amazing purposes. Personally, I am considering buying an R5.
But, Canon has a history of small buffers in the 5D line. About 2 to 2.5 seconds. If that holds true, then, in certain circumstances, I will likely turn off the SD card and only write to the CFE card and the 12 fps spec (and 20 fps spec for that matter) should be able to be continuous. So, in a way, this is a very nice compromise in that you do have the 12 fps continues by turning off the SD card or you do have the opportunity to write to two card slots but the speed will slow after the buffer is filled. For me, I would have preferred two CFE slots, but this is not the end of the world.
But, UHS II cards cannot keep up with the specs of the R5. CFE can. So yes, we have gotten to the point where UHS II cards are too slow. This is why you have the CFE, XQD and other formats. As for "our" needs, that really depends on the photographer. But, if you track complaints people have about Sony cameras, pretty high on the list, at least that I hear, is that if you rattle off a series of shots, it takes a fairly long time for the buffer to clear to their UHS II cards and the camera is frozen during that time. Granted, Canon let's you write and do things during that time. One of the many reasons I like Canon.
As to the importance, as I've seen a few comments go by on this, all I can say is slower fps have impacted my photography far more than DR ever has.