Lets put this in perspective for you.
Sure, sure.. the mini is indeed 8 years newer and therefore a lot better, and it has to be. But that was never the point. The point is NOT to put that 5K+ on the counter and spend it instead for a dive trip to Indonesia while keeping the cheesegrater and do the 16th little bitty upgrade for Mojave with a BIG grin on your face....
That's the reason why the cheesegrater is so sexy.
But hey, let's talk about your new shiny mac mini from 2018 in a full whopping eight years from now in 2026. I humbly invite you to zoom yourself into the year 2026 with a non-upgradable fully enclosed mac mini with T2 chip for controlled product obsolescence. Will you be able to postpone the next computer upgrade for another two to three years well into the year 2029 and do a dive trip instead?
This is basically where we cheesegraters do stand at the moment in the year 2018 from the viewpoint of the year 2010.
The ugly truth here is this: From the viewpoint of Apple, they absolutely hate us 5.1's They do not want their users to be able to use a computer for such a long extended time and be cool with it. Over the last 6 years, they degraded their computer product portfolio to "utility device status" to quicken the sales cycle and shorten the usable lifespan of your price increasing Apple products. They deleted all upgrade possibilities. The intentional stopping of e-GPU support for the MP6.1 or the battery manipulation software on your iPhone is just an example. There should be law suits...
In your case, you did not have much choice, because a real great product such as the 5.1 is just not available any more these days, -and never will be again. The test of time will show, how happy you will be with your mac mini in the year of 2026. I hope you don't sit on a mountain of controlled product obsolescence landmines.
If you want to know how companies think these days, watch this down below. It gives you a good primer.
But then again, did you really have free choice in the first place?