The purpose of a people group list (e.g. Joshua Project, World Christian Database, PeopleGroups.info) is not to deliver the most hyper-accurate census of all of the ethnicities in the world. That would be virtually impossible. No one - not even the people in the people group - precisely agrees on where the boundaries of the people groups lie. When, for example, does a Somali stop being a Somali and start being a Somali American... and when do they stop being a Somali American and just be American, much as many other peoples in America have done? We can debate (hotly!) "when" it happens (and "when it should" happen).
More, the demographics of people groups are changing all the time due to birth, death, immigration and emigration. So, just like a census, it is an order-of-magnitude estimate of where the people group is in a given year--but it can't ever be accurate down to the last digit.
What, then, is the purpose of a people group list?
One is to provide strategic intelligence about peoples and places: to explore what makes a people group in this place different from the same group in other places and from other groups in this place. To explore why this group is reached or unreached: what factors were strategic opportunities and barriers. Not to describe a group in extreme detail but to provide actionable suggestions for opportunities for the Gospel.
Another is to provide a "to-do" list. Every people group on the list is equally valuable in God's eyes. Making certain that the Great Commission is fully completed in each group (presence, blessing, witness, proclamation, discipling, gathering in community, baptism) is critical. Some groups are further along in this process than others, and so have different elements of the task "left to do."
Various agencies, like parts of the body, are equipped to minister to different peoples in different situations. We need the whole world to reach the whole list.
Much of the debate about unreached peoples lists centers around "which are unreached" - that pesky on/off switch - because we have a perception of limited resources going around, and so we have to decide where to allocate scarce quantities of money and missionaries. While I acknowledge this present reality, I suggest the better route in the future is to devote some precious resources to changing the resource pool in practical, achievable ways, so that we have more manpower to reach the total list. (The appropriate manpower for specific entries on the list is a different subject, and one I've started to tackle here and here.)