There isn't really such a thing as an immigrant visa. These non-immigrant visas are the only legal route to come here, by and large, excluding a few obvious exceptions like marriage to an American.
Also, it's quite hard to become a permanent resident/immigrant even without the obstacle of this being categorically prohibited. My family, for instance, overcame some very low odds of success to make this happen (highly educated, both PhDs, for what it's worth).
I have learned that most Americans, probably through no fault of their own, have absolutely no understanding of how their own immigration system works. The options for legal immigration were _extremely_ limited and byzantine, and have been for decades, long before Trump.
That's interesting. European countries do have immigrant visas, and I think Canada does too. (As in, a visa that's issued for the sole purpose of letting you immigrate.)
>> excluding a few obvious exceptions like marriage to an American.
this is a good example, because let's say someone is here on a student visa or temp work visa, falls in love and gets married. without the ability to adjust their immigration status they now have to leave the country - probably for years - to apply and hopefully get a greencard. Good luck making that marraige work.
The US has three classes of immigrant visa. See the bottom of the state department visa resources page.
This is what is broken. The current system is archaic and circuitous. It also performs a legal fiction around non-immigrant visas functionally being a path to permanent immigration.
We should increase the number of immigrant visas and make it straightforward what the process is to get a green card like what one would see in other countries like Canada and Australia.
Meanwhile, non immigrant visas should remain non immigrant and very restricted criteria for changing status (eg. marriage) without reapplying abroad.