I don’t think you’re reading my post right. Download an xml before you begin the fortress and then start it and then look up any migrant arriving in Legends Viewer (even if it was downloaded 20 ‘years’ before) and they will have existed. They can marry, they can give up professions, and so on. Some have extensive (if mundane) backstories.
You can do that if you wish. I don't think I ever claimed that none of the immigrants did not exist before they arrived at your fortress; migrant waves are a mixture of pre-existing historical characters and characters conjured out of thin air. You can tell at a glance in their bio which group the migrant belongs to and finding a particular migrant wave consists of nothing but historical characters would not prove anything in general terms.
Whether or not they have existed or not really depends on the age of your fortress and the world you're playing in. At first you get histfigs with extensive histories, but after a while they run out (or can't be bothered moving to your fort) and you start getting history-less dwarves genned from abstract population numbers. The latter will only get spots in legends mode when they are turned into histfigs by moving into your fort.
Either way I wouldn't see this as much of a problem, as ShonaiDweller said, it shouldn't be too difficult to conjure immigrants of other races from the abstract population numbers that already do exist for each site.
Regarding the problem with soap makers, I imagine it will be looked at whenever a economy rewrite comes around, as then it will have to keep track of the amount of each profession in a given site, and balance it out to make sure the economy works. You can't have a dwarven hillock with only soap makers, so the amount of soap makers should decrease (and probably lead to the amount of farmers increasing drastically but I digress).
The problem is they don't come from abstract population numbers, they are conjured into existence out of nothing, we know this is the case because it is possible to have large migrant waves when dwarves were literally extinct in your world prior to creating your fortress. None of those migrants will be historical characters but there will be no shortage of them.
Since the non-historical migrants are not taken from any existing site population, there is presently no way for us to determine the proportion of non-dwarves in our civilisation in general. We are given dwarves because the dwarfness of our civilization is an established abstract fact that applies regardless of whether there are any actual dwarves in the world at all or if our entire civilization consists of goblins.
The same problem applies to the economy, we will not be able to determine the correct proportion of cheesemakers because the non-historical migrants don't come from any actual location, unlike the historical ones. Your not creating people based upon a site's information, you are creating a typical dwarf of civilization X.