IMHO, it's not healthy to be 100% invested in any asset - treasuries, stocks, commodities, even cash. The YMOYL recommendation may have made sense at the moment the book was written, but markets change in unpredictable ways and diversification is a critical preparation for an unknowable future.
BTW, I own long term treasuries as a key part of my portfolio. They're great as a counterbalance to swings in the stock market. I'll always keep a set percentage - if they fall in value it most likely means my stocks are doing great and I'll just rebalance with those profits.