Unlike the terf objections, which I see all the time, I haven't seen a non-binary-identifying person object to "cis," but I believe you that it happens -- and it makes no sense to me. I say this as a person who identifies as somewhat genderqueer myself. In the broader world, most people assume I'm cis (whether or not they're using the term); among people who know me well, most know that I don't feel quite entirely cis. I receive a lot of cis privilege but not 100% of it. Cis is a term we absolutely need.

To use one of your examples, would it be better for bi people if "het" did not exist as a concept, and there were "gay" and "lesbian" but no term for the other side of that binary? We use binaries even though they're incomplete. We need to. In some ways, we need to grow away from them and allow more options, but in other ways, there are times when they exist and are the actually right thing.