Your point is certainly well taken. As a volunteer with the MIA Recovery Network, I would just like to mention a few things. Please note that there is also an acronym, FOD, in dealing with World War I and World War II Missing. That stands for Finding of Death, and in the case of the army, that date is one year and a day after the MIA date. The airforce has its own way of determining the KIA date in cases of MIA's and I've yet to figure out a system. My husband's uncle has an official death date that is actually his FOD ,a year and a day after his MIA date. Although this is the official record, in reality, he died a year and a day earlier. But all records list that FOD date as his date of death.
Now, in regards to your question. Someone can still be missing and be listed as KIA. In the European Theater, if a person physically witnessed the death of a soldier, and that soldier's body was never recovered, that person has a listing of KIA, because there was witness to the death. (This doesn't happen in every case, but it is a frequent occurrence.) But, that KIA is still missing, thereby, he is also MIA.
And, in reality, after the FOD was established through official investigations that lasted into 1952, then the soldier was officially considered to be KIA. So, in reality, all of those who are listed as MIA have been determined to be killed in action as well.
If someone died in captivity and the body was never recovered, those men are also considered to be Killed in Action, despite having been prisoners, because they were in an official conflict area at the time of their death. In Cabanatuan, for instance, there were mass burials of deaths due to dysentery. Many of the bodies were co-mingled when they were recovered, making it impossible,at the time, for individual identification. They are listed as MIA in addition to KIA, because even though authorities know that these men died, there is no body to send home to family. But these men are buried as unknowns in a military cemetery since they have not been able to be officially identified.
I hope that this answers your question regarding the status of MIA in respect to KIA.