I think his optimism comes from what he has experienced as an average annual growth over the life of the market, I haven't checked the figures myself though.

I often wondered why Dave Ramsey wasn't more popular with HN, he promotes starting businesses like a startup needs to start, work on the side, build your business logically, work your tail off and run LEAN (though he goes a little further and says pay cash for everything).

Plus he doesn't take inflation into account