It's not really disappearing because it isn't really gendered. There are trans mascs who hold onto the lesbian label even after transitioning. There are plenty of non binary people who embrace it too. Typically those who identify as lesbian are more likely to be trans positive and left wing than their gay identifying counterparts.

Lesbian really means something like, "person who society sees as a woman who isn't attracted to cis men." Its actually quite inclusive. This is all just my experience though.

Edit: it's also important to preserve lesbian because of how different our experiences are to bisexuals. Not being into men in our world is quite a trip