GG
There were three aspects to your post including the personal, that one of your sons is an emigrant , and I am very sorry you considered my post patronising as I genuinely feel for the pain of you or any parent in that situation, especially when the country is far away. It must be dreadful.
However I disagree strongly with your view that having another child or more remaining in one's country doesnt provide some relief of the pain, as Azie's comment above supports. Of course you cannot replace the distant child but others and their children can help to fill the empty space left by their departure. I cannot believe you wouldnt find it harder still if your other son was also to emigrate?
On your historical point however of migration in the past no I would not be 'surprised at how often it happened' (far rarer than today is the answer). Youre being rather 'patronsing' yourself here.
The word 'often' is abstract and relative, and maybe you at some point learned about migration in the past and were surprised to find it more frequent than you had thought, but that is not my perspective. I have been aware that it occurs as long as I can remember and also aware that it was never anything like it is now!
Think about it GG, if it has been widespread then global culture and languages would have become far more widespread far sooner and national and local cultures languages and dialects lost. In fact after the initial 'peopling of the world' the vast majority pretty much stayed put until the age of exploration and the colonisation of the new world, and then the repeopling of that from Europe took place but it was still very much a minority activity (with some exceptions of forced migration such as the potato famine and pogroms).
Incidentally nor did I say I am not interested in my family history, I did say I am not especially interested in the experiences of my two distant cousins who went to Australia after WW2. I am very interested in family history, though more the earlier stuff than post WW2.