To print a full size photo on a Canon PRO-4000 (44"x66") at 300ppi, you would need a 261 megapixel image. Now, granted that's not realistic, but 300ppi is generally the gold stand for printing resolution.
A 45mp sensor only gives you an anemic 51ppi when printing that large. So yes, when printing large there is a legitimate reason for having those high resolutions.
Now granted, before people jump all over me with the "but the images still look fine when viewing from a normal distance" argument, yes, I admit that they do look ok. I have printed an uncropped image from my 5DIV at 44"x66", and it does look ok at a normal distance. But really, it's a little lackluster. If you were printing a photo of a city skyline or something with a lot of detail, it would be really neat to be able to get up close and see the small details, but you can't when the initial photo was only 30mp, or even 45mp.
I do my own printing and own a 44" Epson printer, so I have some experience with these things.
We most certainly do NOT require native 300 ppl for high quality prints. We do typically interpolate the original image to 300ppi (or, as some prefer on Epson, 360ppi), but we can interpolate from much lower native resolutions and produce excellent quality in very large images. Many regard something around native 180ppi to be the lower threshold.
I don't know where you came up with your 51ppi number.
I don't have the pixel dimensions for a theoretical 45MP sensor handy but a 5DsR with 50MP has 8688 photo sites on the long dimension and 6792 on the short. You aren't going to make a 44" "tall" print on a 44" printer, more likely 40" with a typical margin, so let's look at a (huge!) 40" x 60" print. 8688 pixels divided by 60" is approximately 150 ppi uninterpolated — or three times higher than your 51 ppl number. Now a 40MP or 45MP sensor would produce a slightly lower native ppi value... but not that much lower.
In any case, a rumored 40MP or 45MP camera is almost certainly NOT the "mirrorless 5DsR." If it exists as described, it is far more likely the "mirrorless 5DIV." That makes logical sense for a whole bunch of reason, including that Canon is not going to decrease the pixel resolution of their high resolution model.
If the rumors of THAT camera having 75MP or a bit more than 80MP are true (which is likely given the MP resolution of the most recent Sony high MP sensors), then the math does get us to and beyond that minimal 180ppi standard for a 40" x 60" print... which is quite astounding for the full frame sensor.