Whether you propose it or not, they will fight it. And they will get citizenship through other means (such as ancor babies and chain weddings).
> Nor that such welfare issues be up for a democratic vote
You know that in many countries illegal immigrants protest for welfare benefits (even though they are not citizens). Whether you like it or not, they will claim it.
If an immigrant has cancer, you cannot just send him away from the hospital.
> You've argued that permitting highly-skilled immigration is beneficial; but not that restricting low-skilled immigration isn't. What's the economic argument for trade barriers on low-skilled labor?
The fact that low-skilled labourers are often a net-expense to most governments (i.e. more tax money is spent on them and their social ills).
The lower strata of society is also more likely to be involved in crime (which affects the rest of the population) and have explosive population growth. The latter is important since in 99.99% of countries, children born in that country gets citizenship.
I don't propose that states automatically grant welfare to any visitors; it's free movement that I claim is a fundamental right, not free money. Nor do I propose immigrants have automatic citizens' votes, not in any immediate time period. Nor that such welfare issues be up for a democratic vote -- they could be a constitutional issue instead.
I don't see how, with these qualifications, this leads to a collapsed welfare state. Without welfare guarantees (at least not for new immigrants), there's no selection tendency for rent-seeking. Instead there's selection pressure for hard-working people, living in states where their efforts are undervalued (because of broken government or broken economy).
On a side note, I believe that immigration discrimination is good. It allows minorities who are productive members of socialist societies (i.e. tax heavy) to flee in to countries where their skills are valued.
I think you've misunderstood your argument. You've argued that permitting highly-skilled immigration is beneficial; but not that restricting low-skilled immigration isn't. What's the economic argument for trade barriers on low-skilled labor?