I think a very low percentage of migrants do it so that they can get a better passport for traveling. Sure, there are people who do that also.

Fair point. I also think the same group you're talking about is probably not thinking about going back home with just government benefits to sustain them. They probably are more focused on working in countries with higher wages and building big homes back home. I think it tends to be more wage driven than government benefit driven, but I could be wrong.