Are you sure?
First, no known source mentions of Slavic invasion into Poland.
Second, Celtic and Germanic tribes, which left Poland around 400 AD, weren't so numerous group, there is very little percent of haplogroups R1b U106 and I1 in Southern Europe and Northern Africa. (no more than 5-10%)
I think you're making a number of leaps here. First off I don't think that such a solid case can be made certain Y-Haplogroups being "exclusive" for a certain ethnic group (for instance,
U-106 seems to have been in Britain before the invasion of the Germanic Anglo-Saxons). Second, its highly doubtful that there were any Celtic speakers left in that area around 400 AD. Celtic languages disappeared from Central Europe between the Gallic Wars and the Marcomannic Wars, and by the onset of the Migration Period, Celtic languages were largely restricted to the British Isles and the atlantic Gaul (what was to become Britanny). Only the Cotini in the western Carpathian region seem to have lasted a bit longer.
Second, by the start of the Middle Ages, Slavic tribes were found substantially further west than the modern (post-1945) borders of Poland. Instead, Slavic tribes lived on much of territory of former East Germany (east of the rivers Elbe and Saale), and even the eastern parts of modern Schleswig-Holstein (where you have overtly Slavic town names like "
Lübeck" and "
Grömitz"). In Roman times, these areas were all inhabited by Germanic tribes (including the Langobards and the Suebi for example), so its very clear that some form of migration happened.
Further, its implausible that the Proto-Slavic homeland was on the area of modern Poland. In particular because in that case, I find it implausible to explain where the other branches of Slavic (East Slavic and South Slavic) come from. For East Slavic, the matter is especially problematic since you then require a migration
west-to-east, against the general migration direction of the Migration Period. If you argue that the division of Slavic into its sub-branches (West, East and South) is actually older, then this stands in contrast with the internal linguistic evidence of the Slavic languages (which suggests that the breakup didn't occur before the Migration Period). This is why in my opinion - we should be placing the homeland of the Slavic languages somewhere in the east, either the area of the Chernoles / Zarubintsy cultures, or in the area of the Milograd culture, as the most likely answers. If we exclude the other candidates, what is the most likely correct answer?
Of course, 30 percent of Polish people have nowadays these groups, but some of them can be descendants of German settlers in Medieval ages.
So I think that proportions in ancient times were similar - around 40% people were Celtic and Germanic and 60% - Proto-Slavic with typical haplogroups R1a1 M458 and Z280. After migrations of Celts and Germans rest of people were slavicized.
I would actually agree that whatever was left of the population of the Germanic tribes at the conclusion of the Migration Period (and archaeology says that the population decreased sharply) would have become Slavicized. The linguistic evidence is verymuch in favour of that, as in the Slavic languages, you have in particular words that relate to natural environment, animal-keeping and trade that are borrowed from Germanic.