The problem of course, given that the topic is "source of wealth", is how well gilts and treasuries would work for someone starting out now. In 2061 how many people will be attributing their wealth to going all-in into fixed income in 2021?
1940's to 1970's saw bond yields rising. A Permanent Portfolio with 50% in a 10 year ladder, 25% FT All Share and 25% Precious Metals (silver pre 1972, gold since 1972).
In nominal yearly gains/losses terms, the first chart in the above clickable thumbnail, I'd say it would be pretty difficult from those bars to identify periods of low/rising or high/declining bond yields, despite the portfolio holding 50% exposure to such bonds.
The PP is of course a conservative 25% stock portfolio, the lower level of what Ben Graham suggested, however its primary intent is for "the money you can't afford to lose" - as Harry Browne proposed it to be for. Whilst still churning out modest real gains (3.5%).
What might be said for a assumption of present high bond prices might equally be said for stocks and gold. There's no guarantees that all-stock will do well over the next decade or more, historically stock total returns have seen a decade where -75% losses were endured.
Similarly some opt for Corporate Bonds, but their premium over Treasury bonds reflects the default rate. Whatever additional gains you might make from their higher yields could be given back via failures.
100% all in bonds, or stocks, or gold ... or any one single basket is a significant risk factor. Source and preservation of wealth is better placed through diversification. Yes one asset will likely do well, others poorly, so there's always a element of regret that if only you'd gone 100% into X, Y or Z how so much better the results would have been. But equally 100% in the worst asset will likely have been most painful, if not critical.
Continuing to remain diversified, including holding some bonds over periods when yields were rising is better than placing a all-in one asset bet.