Most proponents of UBI see it as, at least eventually, replacing some range of existing government means- and/or behavior-tested benefit programs and, possibly, some age- or disability-tested ones (in the US, most advocates would probably include, at a minimum, EITC, General Assistance programs, SNAP, TANF, Section 8 housing subsidies; some would also include Child Tax Credit, Tuition and Student Loan related credits and deductions, Medicaid, Medicare, and/or Social Security.)
I've never heard any advocate argue that it would replace all social services.
Most proponents of UBI see it as, at least eventually, replacing some range of existing government means- and/or behavior-tested benefit programs and, possibly, some age- or disability-tested ones (in the US, most advocates would probably include, at a minimum, EITC, General Assistance programs, SNAP, TANF, Section 8 housing subsidies; some would also include Child Tax Credit, Tuition and Student Loan related credits and deductions, Medicaid, Medicare, and/or Social Security.)
I've never heard any advocate argue that it would replace all social services.