Illness? as in a condition that causes harm and/or stress? Then if someone one is uncomfortable with being treated and living as a cis gender individual, then the logical cure is to let them transition and live as the gender that fits them.
So, if people dismiss transgenderism as an illness, that is literally someone siding with the problem not the cure, and actually causing problems in society for a group of people that are just trying to lives their lives.

There was also a time when homosexuality was considered a mental illness, or being left handed, headaches were also once blamed on evil spirits.
Contemporary society has evolved beyond a point where gender dysphoria is considered a mental illness.