The safety valve is not the sole reason, and yes a fair share are making their way to North America. The difference between Ostafrika and the United States in this case however, and Canada unless I've read the homesteading laws wrong, is that the Poles who travel to North America would be left solely with what they could carry until such a time they could become citizens (and little especially fertile land was left by that time in the USA at least), whereas in Ostafrika they could file applications straight-away as German citizens and potentially petition for further monetary support from the colonial authorities in establishing their economic well-being.
I cant speak to the States, but I do know Canada was settled later.
My ancestors came from both Austrian Poland and Prussian Poland. My prussian ones actually settled in Montreal first before moving west, while the Austrian ones went straight to Saskatchewan.
See here:
https://en.m.wikipedia.org/wiki/Last_Best_West
It was the last best west program.
Also, the land settled in Canada was actually very fertile along the Saskatchewan rivers (the growing season is shorter, but its much less arid).
Moreover, the Canadian system not only picked up steam after the Americans "closed" the frontier, but the homesteading worked slightly differently in Canada.
The following is from the wikipedia article on the Dominion Lands Act.
An important difference between the Canadian and U.S. systems was that farmers under the Canadian system could buy a neighboring lot for an additional $10 registration fee, once they had made certain improvements to their original quarter-section. This allowed most farmsteads to quickly double in size, and was especially important in the southern
Palliser's Trianglearea of the prairies, which was very arid. There it was all but impossible to have a functional farm on only 160 acres (0.65 km2), but it could be managed with 320. Canadian agriculture was consequently more successful than U.S. agriculture in this arid region.
[6]
Bloc settlements were encouraged by section 37 which allowed associations of 10 or more settlers to group their houses together to form a settlement to fulfil their cultivation obligations on their own homestead while residing in a hamlet.
So not only could someone increase their land, it was easy to bring over communities or extended clans... which is what my family did.
This is the Austrian side, but one brother came, then paid to gave his other two brothers come over. This ballooned to the other twelve brothers and sisters, and their spouses.
The bloc settlements also allowed communities to maintain their languages and culture. My grandparents were born in Canada in the late 20s, and their home language was Polish. Its why you can still find Polish or Ukrainian speaking churches in the Prairies to this day.
The point I want to make is that when the American frontier closed, the Canadian one boomed.
There was always an outlet for east european farmers.
Especially for Canada, during the Laurier government years, it was policy to recruit East/Central European 'peasants'.
This iswhat the minister of the interior said during those years of the immigrants he wanted:
"When I speak of quality I have in mind, I think, something that is quite different from what is in the mind of the average writer or speaker upon the question of Immigration. I think a stalwart peasant in a sheep-skin coat, born on the soil, whose forefathers have been farmers for ten generations, with a stout wife and a half-dozen children, is good quality."
That quote gets seen in a lot of Canadian textbooks.
As another source, see here:
http://www.british-immigrants-in-montreal.com/clifford_sifton_policy.html