Truth be told, there are plenty of trans women who identify as lesbian, filling whatever tiny whole trans men may have left. No, lesbians aren't disappearing, unless you don't count trans lesbians as lesbians, so ultimately that argument is based on transphobia.

Furthermore, many trans men are gay, so they were never under the label of lesbian in the first place. Being trans isn't just queer people trying to gain privilege under another gender label, such as taking on a heterosexual identity by transitioning. Once someone transitions, they are heavily discriminated against so they lose more privilege as a heterosexual trans person than a gay or lesbian cis.