You can have open borders or a welfare state. You can't have both.
I for one, will always advocate for open borders.
The most apt comment on this subject, by far. Bonus points for the brevity.
It is one thing to encourage immigration when viewed in its original context, which is to have people come and help settle the land. (Though the intent of the immigration-friendly NC appears to be, to use bajo_cero-style lingo, racist and genocidal: see below).
It is entirely another thing when people are coming en masse with the clear and stated (or unstated) intent of taking advantage of the existing welfare state, which by definition cannot be a blank check to the population of the world. That just doesn't work.
When the NC was written with its open borders, it was not predicated on the assumption that the government (=the existing taxpayers) will be subsidizing said immigrants' stay here.
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Regarding squatter's rights vs. usucaption:
Per Wikipedia, usucaption, at least as conceived, is a mechanism for filling in holes in procedure regarding a sale/transfer of property that actually took place. When there is little serious contention that anyone ever purchased the property from anyone, that is called adverse possession, aka squatter's rights. And while it is true that squatter's rights may be legally valid and even result in full title to the said property, clearly they are valid in a "too bad!" sort of way. Which is worth bearing in mind when considering similarly "tough-luck-it's-legal" remedies. Such as, say, eminent domain.
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Finally, one other point: the immigration guarantees by the Constitution, are not
quite as neutral and innocent as one would assume simply by hearing about it second-hand. Even a cursory reading of the constitution finds the language - retained from the original 1953 version - that "The Federal Government will encourage
European immigration". Why specifically European? Perhaps because the main aim of the authorities at the time the NC was drafted was to bring the territory and indigenous populations under the state's control, and any and all European immigration would help to that end. (And this is not an issue that lacks relevance today: the Argentine indifference/hostility to indigenous peoples' (for example: casique/Qom) issues - except when electorally convenient - has continued up to and including Macri's administration, and notoriously so in the Kirchner era).
Something to have in mind when you are accused of racism for contemplating that eliminating a gigantic slum from the very center of the city just
might be a good idea on many levels. And on the other hand, something to bear in mind when applying the spirit of the original constitution to today's situation. There is a big difference between my owning a huge tract of land, far bigger than I could do anything with myself, and inviting other people to come help me settle it, and on the other hand taking any permissions I may have given at the time as carte blanche for anyone and everyone to enter my home, help themselves to what's in the fridge, urinate on the beds, and claim title.