I personally am very binary - I thought of myself "100% masc" (albeit questioning in significant ways) for quite a while, and then in figuring out I am trans I now am "100% fem", and the guts of my gender coin-flip (replace with whatever binary metaphor you like) happened as quickly as I could manage. But I absolutely recognize a gender spectrum that is at least two-dimensional and possibly 3D. Much as pretty much everyone agrees that there's a lot of folks that are in between 100% homosexual and 100% heterosexual (pan, bi, and all the rest) and with a y-axis of way into the nympho (hyper?) sexual and down into the a- or demi- and so on. Me being binary in my own gender doesn't preclude nonbinary or agender genders any more than me being ace or Catholic precludes heteroflexible people or Protestants, Buddhists, atheists, or agnostics from existing (though the religious example has the significant hole of everyone thinking everyone else is actually wrong, I suppose).
Want to eliminate gender? Go ahead, it's a social construct, right? Eliminate race and ethnicity too please, while you're at it. But I will still be happy when skirt goes spinny, even with people yelling "clothes aren't inherently gendered" to my face. When my great-grandchild grows up in a post-gender world, xey can have a cisgender-style mindset and maybe that'll be okay too.