We lose knowledge, diversity, skills, and a link to our past. I understand why the people are leaving, I would probably make the same choice. But we shouldn't also act like there isn't a loss.
I guess whether this is a big problem depends on what you value. From some perspectives, knowledge and skills are not ends unto themselves, but only have utility in a specific context. For example, I care not that the skill of carving elephant ivory is basically a lost art at this point. Diversity? I can't say that the presence of nomads in the desert was really moving the needle on that very much in my daily life. A link to our past? Perhaps, but, as GP pointed out, only in the sense of a living museum exhibit. It is not like many people actually go to encounter this link personally, and in any case, even if they did, these are people.
I admit a certain amount of wistful nostalgia, but I personally don't consider nostalgia to be the same thing as sadness. Nostalgia is most often a delusion, not a genuine recollection of the good old days.
You're taking a very utilitarian view of these things. Nothing wrong with that. But many people enjoy knowing that there are still those who stick by their old ways without there being any tangible benefit to themselves or the rest of society.
While this is true, this process has also happened in our own culture, albeit a few hundred years ago.
In the UK the industrial revolution and agricultural mechanisation caused a huge migration from the countryside to the cities. Today, you can still see plenty of ruins of old settlements, cottages, churches, etc. which were abandoned and left to decay. In many parts of Scotland, you can walk to shielings, which were housing for herdsmen during the summer months. They would migrate there with their cattle and stay up in the hills, and migrate back down during winter when it was too inhospitable. They would have had a fairly boring and miserable existence. Living in relative isolation with no company but the animals, all food having to be brought in, and then long walks to the towns and cities to sell them. Today, cattle are kept in fields, fed with grass and cattle feed, and moved around in lorries. While there's a certain romanticism for the migratory lifestyle, and it might even be novel and interesting for a short while, I don't think the drudgery in the long term is something I could tolerate.
