I'm not sure what the point of your Social Security example is. Social Security works nothing like that currently. Social Security is your money. What you take out when you retire is based on what you put in, no matter how wealthy you are. It is not a welfare program. If you never work, then you are not eligible for Social Security.

As far as UBI replacing welfare programs, I would be ok with that as long as it truly replaced all welfare and did not result in a net increase in taxes (once factoring in the UBI itself). As you say, the gross cost would be much higher because everybody would get the UBI. How that effects an individual taxpayer depends on the details. Other than possibly simplification (and again, this depends on the details), I don't see any real advantage over existing welfare programs though.

Whether or not UBI is more expensive than existing welfare programs depends entirely on how big the UBI is vs. the outlay of existing welfare programs. It could be much more expensive, much less expensive or the same depending on the implementation.