> A UBI in Europe could mostly just replace current spending at similar levels

While UBI is more administratively efficient than means-tested benefit programs, it also inherently reaches a lot more people with its maximum benefit. I don't think there is any major country (there might be some small ones with a lot of money flowing in from the outside) that could sustainably keep UBI at any comfortable level rather than an alleviating severe poverty level at this time.

Of course, on average you'd need to recover the full UBI from the average (not median) person via taxes.