The BC isn’t destroyed - just sealed. This is from NC and I assume most other states have the same procedure.
VIII. AMENDMENT OF BIRTH CERTIFICATE
A. Provision is made within the adoption statute, N.C.G.S.§48-9-107, for changing the name of the child and for the preparation of a new birth certificate in the adoptive name. The original birth certificate is sealed and a new one is issued by the Office of Vital Records in the state where the child was born. This is done upon receipt of the Court’s report that the Decree of Adoption has been issued and that the change of name is authorized. This Report, which is actually an order issued by the Court of Adoptions, is made on Form DSS-1815, Report to Vital Records. The Clerk of Court signs the Report and forwards it to the Division of Social Services at the time the Decree of Adoption is sent.
B. In North Carolina, as in most states, the new birth certificate makes no reference to the adoption, and the names of the adoptive parents are shown as though they were the birth parents of the child. The original certificate is placed under seal which may not be broken except as provided in N.C.G.S.§48-9-105 and 106 for the opening of the adoption record.
C. N.C.G.S.§48-9-108 provides for restoration of the original birth certificate if the decree of adoption is set aside.
http://info.dhhs.state.nc.us/olm/manuals/dss/csm-50/man/CSs1302-08.htm
I think this type of legislative provision is out of date. In a time when being born to an unmarried mother could have a substantial negative impact on the child, it made some sense. However, today, that’s not an issue. It seems to me that the good of the child is best served by specifying the natural parents as the natural parents, and the adoptive guardians as such.