How about just piling up cash so that you have a few years' worth before pulling the retirement trigger, with the rest in equities?
Then if we face a prolonged downturn, you can (a) spend only 4% of the current value of your portfolio annually and take up part-time work to cover the rest, first drawing down cash before selling any stock; and/or (b) lower expenses as much as possible
Why exactly does the early retiree need to invest in a lot of bonds or TIPs?
Inflation. Cash loses buying power due to inflation.
So there's a tradeoff. TIPS at rates negative compared to inflation might strongly outperform cash for "a few years".
Example: 2% inflation x 3 years = 6% lost value in cash. TIPS that got nominal 1.5% annually during the same time would lose 1.5% of buying power value (half a percent times three years) but beat cash by 4.5%.
Regular bonds might also beat cash, but the details are different. TIPS vary based on inflation, giving shelter to that factor - in exchange for rates that are usually lower than regular bonds. Regular bonds often beat TIPS, therefore beating cash even more.
That said, bonds (including TIPS) also can gain or lose value due to changes in interest rates. Depending on events, this could mean that bonds and TIPS perform better or worse than cash during a specific period. Cash avoids this uncertainty, but at the expense of most often losing value due to inflation.
Holding cash could be viable, but often is non-optimal. Tradeoffs!