You can have a welfare state or you can have multiculturalism. You can't have both.
Welfare states ALWAYS self-destruct eventually, whether Rome or modern ones. Welfare allows the non-working to have large numbers of kids, most of whom will not grow up to be taxpayers. Meanwhile, the middle class can't afford to have more than one kid, because they need both spouses to work to pay the tax burden.
Allowing immigration of the world's underclass just makes the collapse faster.