I have never been able to get my head around the issue in the stream of thought that the loss of pixels upon cropping denigrates the resolution, quality of those pixels remaining in the image notwithstanding Burk's good explanation and PHRubn's following comment.
To make an analogy:
I have a jar of sliced peaches which I purchased because I like its taste (resolution) - the label advertises (x) number of slices (pixels). I dish out a serving and the reduction of the number of slices (pixels) does nothing to the taste (quality) of those slices (pixels) remaining; the taste (quality) is unchanged from what it was before the jar was opened. Another - I have a piece of 8x11 1/2 paper which I trim down to 6x8 - the quality of the 6x8 is unchanged from what it was before it was trimmed out.
?????

You may not have intended it as such, but this is REALLY a trick question!

The honest answer is that you get a 20MP image cropped from a 30MP original file. You're not getting the full resolution of the lens and sensor combination.

Of course, at a certain point, you must re-size the image through software interpolation to enlarge it, but at some point, you dip below the required 240 pixels per inch of ORIGINAL, in-camera generated pixels required to spread over each linear inch of output, in order to meet extinction resolution at 12.8 inches from an 8x10 print.

Extinction resolution is the point at which your eyes cannot resolve any more detail from an image, even if you increase the available pixel count. Most labs will tell you that 240 to 250 PPI — un-interpolated, from the original file — is what is needed to preserve all the detail you can see at 8x10. As the print size and appropriate viewing distance increase, that number is reduced. Conversely, at smaller print sizes, it is INCREASED. (Yes, that's counter-intuitive, I know, but true.)

All that said, a 20MP crop from a 30MP original will make a very large print from most subject matter. 30x40 inches is not outside the realm of acceptability, unless your subject matter DEMANDS up-close pixel peeping (such as a large group of people, or a highly detailed landscape, or a military spy photo). I've made a few un-cropped 40x30 prints from 16MP Micro 4/3 images, and they look just fine when you view an entire print from 50".

In general, people put far too much emphasis on MP count. Thousands of the best, most dramatic and iconic images of the 20th century were made with far less effective resolution than can be had from a 30MP camera.

I once worked for a designer whose motto was, "If you can't make it GOOD, make it BIG." I believe the original source of that was a Time-Life magazine editor. They were known for occasional "double-truck bleed" center-fold photos of important events. Many of those were from 35mm Tri-X negatives, carefully push-processed in soft-working developers like Acufine. Our tools are much better today! Go forth and make images while there is still light to do so!

I have never been able to get my head around the issue in the stream of thought that the loss of pixels upon cropping denigrates the resolution, quality of those pixels remaining in the image notwithstanding Burk's good explanation and PHRubn's following comment.
To make an analogy:
I have a jar of sliced peaches which I purchased because I like its taste (resolution) - the label advertises (x) number of slices (pixels). I dish out a serving and the reduction of the number of slices (pixels) does nothing to the taste (quality) of those slices (pixels) remaining; the taste (quality) is unchanged from what it was before the jar was opened. Another - I have a piece of 8x11 1/2 paper which I trim down to 6x8 - the quality of the 6x8 is unchanged from what it was before it was trimmed out.
?????

As long as we are getting off topic... Statistics and modeling prove nothing. They are alternatives to understanding what is happening in real terms. People rather play with numbers than conducting the experiments which require more work and frustration until you understand clearly what is going on. Next point. Modeling is helpful to rule out alternative explanations or predicting future behavior. In either event, experimentation is the ultimate tool and should not be overlooked.

Can we get back to the OP now?