> Current 30 year bond rates and the performance of stocks in the 21st century are significantly less lucrative than in previous decades.

I'm just eyeballing this, but after adjusting for inflation neither recent stock nor bond yields look out of the ordinary, historically speaking.

There are always reasons to expect growth to go down, and other reasons to expect it to go up. It isn't clear to me why cherrypicking solely from the former group is supposed to be convincing.