"A little more imagination will enable relatively safe diversification."

I agree. Why should anyone accept punishment voluntarily?

Nowadays there are plenty of uncorrelated Liquid Alternatives, which are a much better hedge against such punishment than bonds. Just take a look at the recent excellent article here on CTAs as a proven approach against tail risk without such opportunity costs as from bonds:
https://blogs.stage.cfainstitute.org/investor/2018/04/24/raphael-douady…