So strange to me that when UBI is discussed, pervasive automation is right around the corner to pay for it, but when birth rate decline is discussed, the only solution on the table is open borders. I wonder why that could be?
Because people saying that are doing accounting with money, not productivity.
People need three things to live in retirement. Food, water, shelter[1], and some degree of medical care. We are not in a demographic collapse situation where society cannot be productive enough to fail to provide the first three to the retired. The latter is the only thing in danger of seeing some cuts.
[1] Housing isn't expensive because building four walls and a roof is expensive. Housing is expensive because there's no upward bound on the cost of a square foot of land in a city.