It’s not that complicated. There are people who make a claim that their innate sexual identity is not aligned with their sexual characteristics. It does not matter for my question whether or not their claim is valid. Regardless, this is a group of people who are by virtue of making a claim different from other people who don’t make that claim. So, of the set called People there are two subsets. One is claim-makers and the others who don’t make this claim. Call the first set A and the second set B. All People equal set A+B. Now change the term A for the term Trangender and the term B for Cisgender.
Later we can debate whether the claim is valid. For now, however, we have established that Cis- is just terminology that is a short form label for people not making the claim. Any objection to using this terminology is just contrived. If people really don’t like Cisgender, then they should suggest another short name for this set. Writing out “people who don’t make the claim to have an annate sexual identity not aligned to their sexual characteristics” whenever we want to refer to set B seems a bit cumbersome.