Most displays are 4K or less, and a 4K display is ~8 MP.
What advantage does 45 MP have over 30 MP when images are most commonly viewed on displays with much lower resolution (i.e., the far more common HD monitors or tablets/smartphones)?
How often do you personally upload/share images online that are over 20 MP, and even if you post full resolution images online, how often do you think they’re viewed without downsampling to the much smaller display size being used to view them?
The point is that print is pretty much the only medium for which there’s any need for higher resolution, and as @Sporgon and others point out, 30 MP is generally more than enough for that. For online viewing, even 30 MP is overkill to some exponential power.
That's only if you have control over the subjects you are photographing.
Higher MP bodies allow me to crop far away elements and still get a usable photo, or sometimes you can make multiple photos from the same base photo. Like, I have a photo where I took a 35mm street photography landscape-style pic using 5DsR+16-35 f/4L but I can then crop the RAW to get full quality photos of little scenes of people within the scene. Same thing happens with nature photos, sometimes I take a pic which looked pretty good when I framed it at the time and then realize if I cropped it by 50% it would look much cooler.
Now it's not worth sacrificing everything for this ability, but it certainly is a super useful tool now that I have access to it.
Since this seems to have become a thread about resolution, noise and cropping, I can't resist added a few comments regarding Canon's two most recent top of the line bodies, the 1Dx III and the R5.
In shooting the 1Dx III for almost a year now I have been impressed with the "secret sauce" Canon added to the sensor. The 20mp files of the 1Dx III seem to me to tolerate cropping much better than the 20mp files from the 1Dx II. I don't know why. Some say it is the weaker anti-aliasing filter and I am not knowledgeable enough to argue either way, but I do see a significant improvement between the two generations. I shoot a lot of sports under less than ideal conditions and I have yet to have to reject a photo because it is too noisy or too soft after cropping.
I've only been shooting the R5 for about six weeks but I have been impressed with the files that 45mp sensor delivers at high ISOs. Very little and easily managed noise.
Point being, I'm seeing a convergence in the new sensors, where many of the old arguments about lower resolution=less cropping room and higher resolution=more noise seem to be less and less relevant in my day-to-day experience. YMMV, but I feel as though too many people are stuck in arguments that were valid six or seven years ago, but aren't today. We may have to relegate these arguments to the same dustbin where the lens cap shadow noise arguments have been tossed.
In shooting the 1Dx III for almost a year now I have been impressed with the "secret sauce" Canon added to the sensor. The 20mp files of the 1Dx III seem to me to tolerate cropping much better than the 20mp files from the 1Dx II. I don't know why.[..]