Rare things aren't normal, even if variation is normal. That's what the word "normal" means. If you're trying to prescriptively change what the word "normal" means, it's not going to work. People will use a synonym ("ordinary" perhaps) and keep the meaning the same. You might get a decade where people catch up with the usage, and then the meaning will go right back to what it was, except with language usage altered.

See, for instance, "That's idiotic." No good! "idiot" -> "retarded". "That's retarded." Oops. "retarded" -> "special". "That's special."

What you want to argue for is not that cis isn't normal--it is, and if you change the word "normal" to mean something else, cis will still be the-thing-that-we-mean-now-by-normal. What you want to argue for is to embrace variation. Specifically, you want to argue that this is not normal but it being non-normal is fine and that we have the capacity and obligation to support people when they aren't entirely normal.

Don't attack the words. The words are just the messenger of the meaning. Get at the meaning.

What is normal is that many people are unusual in some way. It's not normal to have Crohn's disease. It's not normal to be fully colorblind. It's not normal to be a super-smeller. It's not normal to be over 6'7 if you undergo otherwise normal male development. But it happens. When it happens, we can either be selfish ("don't fund research into that", "so what if I use a red-to-green color change of the same lightness", "let's perfume the store", "they can just lean back in the driver's seat"), or we can take into account the variation. The more affluent our society, the more we have the resources to make things better for different types of rareness.

The argument needs to be: being cis is normal, and that means that trans people may at times need attention for their specific issues, because their issues aren't identically the same as the most common issues. In a very long train, the squeaky wheel is not normal, but it does, and should, get the grease.