You're right, originated mortgages are slightly different than the full run numbers I put out. But one certainly can pay their 30 year as a 15; I have multiple friends doing just that, at least one that I know is truly doing it in a disciplined way.
I don't see a $500k error. I took numbers straight from amortization tables and investment calculators assuming $x per month invested at a certain rate for some duration. You may disagree on my assessment of the future value of the dollars saved in interest, but they were dollars that were real and were paid from one's account to the bank. Reducing total cash outlay does have some future value as that cash goes someplace.
But the 15 vs 30 bit was for two reasons - first, because so many use the discussion of taking a 30 over a 15 for flexibility... So they can see what it costs, and second, because if instead of the $100 you used, it was a more reasonable amount of money, like $500, then one could indeed pay their 30 off like a 15. In the end, the savings from that point on in terms of returns add up the same no matter what.
I agree that making more is better than less, and that compounding is great. But all I'm saying is that a big return on a little money is not that much, while a little interest on a big principal is a lot of money going the wrong direction. The best way to combat that is get the mortgage gone so that the thousands spent on it monthly can be diverted to doing real and lucrative things. Joe public isn't going to obtain investment real property or do big things with $100 or $500 a month, but if he can free up thousands a month by having the mortgage gone, then that free cash can be converted into doing real and big things, and that's where real wealth will be built.
I'm happy with the approach I'm taking, and I am able to invest, and get rid of my biggest cash outflow ASAP so that I can devote my cash to other things that will have real return and yield for me (besides stock investments). I'm not concerned, I have a very low risk and realistic plan that will have everything paid for and us as millionaires (real, not with some portfolio of bank-owned real estate) far younger than most achieve it. If by some magical stretch I miss out on the compounding of some thousands of dollars today and in the next couple years, I'll not miss it based upon realistic and conservative targets I've evaluated. Feel free to go be mr savvy investor. If it's working out for you, perfect. I know my plan is getting us to the point where we desire and with low risk and great return, so I'm not concerned in the slightest. Good discussion and good luck.