It's a bond.
I Bonds are not negotiable and bonds must be negotiable by definition, so I Bonds are not bonds despite their name. (The second sentence
the wikipedia definition of a financial security says that the word "security" is used vernacularly to refer to any financial instrument even if that instrument is not technically a security, and it works just the same with bonds.) If you don't want to own a T bond anymore, you can sell it to someone. If you don't want to own an I Bond anymore, then just like a nonnegotiable CD you have to put it back to the issuer at terms they decide, using a formula that cares nothing for 'market value'.
I guess you could also say the rate is fixed for 6 months. For the last three 6 month periods, the rate has been over 6.89%, so that's 18 months of DPOYM, according to ERN. And depending on your mortgage rate, the period before that could also have been a good time to not POYM.
If you buy a fixed rate bond (or another fixed rate instrument like a CD) you know you are getting the yield to maturity until the instrument's maturity. Look at three instruments with very similar risk profiles and identical maturities: the fact that the 30 year TIPS trades at
1.5%, the
30 year treasury bond trades at 3.7%, and the I Bond has a 6% current yield (but no meaningful yield to maturity), should tell you all you need to know about inflation expectations for the second, third, and fourth six month windows on that I Bond.
Or for another analogy, if a low quality company is selling off furniture to make common or preferred dividend payments and the dividend yield is
18% that doesn't mean 18% is your hurdle rate and buying anything yielding less is a poor investment, it means that market participants have taken a look at the expected lifetime returns of the instrument and that's the return they need to accept the poor yields in the next year and for the rest of their holding period.
Bonds aside... I think DPOYM or POYM based on SORR risk should be assessed along with time from FIRE. If SORR risk is the main concern, in most cases, DPOYM until you are very near FIRE, then POYM. I wouldn't tell a 20 year old that plans to FIRE at 40 that they should POYM to mitigate SORR.
I would say that sequence of returns risk is
always the primary concern. That's why we have the 4% rule and not the 7% rule or the 10% rule.