My understanding is that for all cause mortality the "safe" BMI is in the 18-25 range with healthspan increasing the closer you are to a BMI of 18.

Even with a perfect BMI and looking totally fit, you can still be at very serious risk of plaque formation. Like people who have hereditary bad cholesterol issues (if it's bad cholesterol due to eating junk food a lot, typically you won't have a good BMI though).

My uncle had a stroke at 52 years old and stayed paralyzed for 15+ years, until his death. When my doctor learned that, the next day I started taking statins.

BTW I'm totally fit, no beer belly, excellent BMI and I look not a bit but much younger than my age (someone even commented that here on HN a few years when they saw a picture of me when I was 49 years old).

And yet I was seriously at risk of plaques detaching and clogging. Statins are not perfect medication but it is what it is.

My point being: even if you've got what science/doctors considers the best BMI in the world, you could still be at risk.