You should really check your numbers if you are going to rant about someone elses. The Nether is the one that is 10,000,000 in each direction, not the overworld. Overworld is 30,000,000. This gives the Nether a grid of 20,000,001x20,000,001 and the Overworld a grid of 30,000,001x30,000,001 ( the origin 0 point is the extra 1 btw. ) The Nethers size is actually 1/9th of the Overworld, not 1/8. You are thinking of the travel relation between the 2, which is not related.
For the record, I was previously doing quick off-hand guesstimates for the numbers. Yes...I was off by quite a ways, but not nearly as much as you were. Plug in the new numbers using your own guesstimates, roundings, and bases, and you still get insanely large numbers:
20,000,001x20,000,001 = 400,000,040,000,001. That's the 2d area.
400,000,040,000,001 x 128 = 51,200,005,120,000,128 total volume.
51,200,005,120,000,128 / 2 for a very rough guess on the amount of Netherrack, and you get: 25,600,002,560,000,064. That's over 25 Quadrillion.
I think there's actually a bit less Netherrack than that, but this varies a lot. Add in Natura from the newer packs, and it'll plummet some more. However, it's still obscenely large. Even at 10% of the volume being Netherrack, you're still over 5 quadrillion.
At 5 netherrack per second, and 10% netherrack by volume:
5,120,000,512,000,012.8 / 5 = 1,024,000,102,400,002.56 seconds
1,024,000,102,400,002.56 / 60 = 17,066,668,373,333.376 minutes
17,066,668,373,333.376 / 60 = 284,444,472,888.8896 hours
284,444,472,888.8896 / 24 = ~11,851,853,037 days
11,851,853,037 / 7,000,000,000 = ~1.7 days for the entire planet
11,851,853,037 / 365.25 = ~32,448,605 years for 1 person.
32,448,605 / 500 = ~64,897 years for 500 people.
All of that is on the massively optimistic side. It can easily be several orders of magnitude higher than that for various reason. Especially once you start to consider real play times, not a perfect 24/7. There is just no feasible way for anyone to come remotely close to mining out the nether. Not even a dent on the grand scale.