When people think about 100mpx being too much, they forget about the bayer filter that essentially means the accurate colour resolution of an image is 1/4 of the total resolution, because each of the 100 million pixels on the sensor is only receiving either red, green or blue light so the colour information from four pixels (at least, more in more complex algorithms) is required to generate the colour for a single pixel. Generally this isn't a big problem, but if you have images with large areas of predominantly one shade, such as a green plant or a red dress, the real perceptive resolution of what that sensor will provide will be significantly less than the 100 megapixels.
So, downsampling a 100 megapixel image to a 25 megapixel image would give you a 25 megapixel image with, assuming an ideal lens, the optimum sharpness and colour accuracy.
Is this important for most people? Of course not. But there are certain tasks, such as high resolution reproduction of artworks or natural objects, where capturing the finest detail accurately is important, and for that a 100 megapixel EOS R can't come soon enough!
Of course at 100megapixels diffraction is going to be a real bitch, but that's another thing altogether!