The first term is relative, the second absolute. If the diet is overall healthy, it's possible it has some bad food. It's less healthy than if it didn't, but not unhealthy.
Also, we have the basic capability for this now. Regular blood testing, etc. Get a physical. Get blood work every 3-6 months. Its just basic stats for your body. I do this. To me it is like checking the oil levels in a car. Why would you not collect this type of data for your body, which is arguably one of the most important "possessions" a person can own?
I'm in Canada, and the idea of routine blood testing for young people (I'm 29) doesn't seem to have caught on.
Unlikely, because how much "less healthy" becomes "unhealthy" is an arbitrary subjective point rather than being something fixed and objective (and because the degree of health impact of various decisions varies widely based on individual factors that aren't all known with any kind of precision, so, even with a fixed objective of 'healthy state', the point at which the average person would reach it and the point at which you reach it may be not at all similar.)
> I imagine we'll get to the point of being able to do this sort of thing in about five years.
I imagine we'll not be meaningfully closer to being able to do this than we are now in about five years. (OTOH, I wouldn't be surprised to see someone marketing something that purports to do this in that time.)
Smoking one cigarette does not make you unhealthy. Smoking one cigarette does make you slightly less healthy.
Think of it as death by a thousand cuts. Lopping off the tip of my finger won't likely kill me, though it does hurt. Keep lopping though, and eventually I am less and less healthy, until at some point, I am dead.