If Satoshi meant 21 as half of 42 for the reasons you're explaining, I don't think that would be all there is to it. He seemed to just see money as a tool, so I don't see him designing a purely focused message of "money is half of the meaning of life." With the 7+7+7, mining bitcoin is a lot like re-rolling again and again on a slot machine. I assume that's at least part of the reference with the 21. It's also common for 21 to be used as a lucky number because of the 3x7 thing by my understanding