I agree that it's a waste of effort, and that the battle is lost even if you had a solid argument, but you don't. "Drone" has been used to refer to unmanned aircraft since at least as far back as World War 2.
Historical argument: Model R/C aircraft have been around for longer (including a video downlink and limited autonomous flight like return-to-home) than the term drone has been applied to them, even if it existed before as you said. Since the capabilities of these R/C aircraft didn't change, it is logical that their name doesn't change either.
Usefulness of the definition: Aircraft which are the size of a van and shooting rockets at brown people in the middle east are also called drone. Since capabilities, dangers and use case differ so radically from a 600g aicraft crashing into trees because it's fun, it is logical that they have a different name.
Political argument: Because drone is associated with the aforementioned military aircraft, it makes it easier to call for more regulation of hobbyists by playing on the public's fears.
> Usefulness of the definition: Aircraft which are the size of a van and shooting rockets at brown people in the middle east are also called drone. Since capabilities, dangers and use case
And yet, in that very argument you used the word "aircraft" to refer to military UAVs. That word "aircraft" is also used to describe vehicles of vastly different designs, sizes, and capabilities, but I don't think there's much confusion caused by this term.
It's okay to have words that describe large groups of things, along with words which describe more specific subsets of groups of things.
This article is a good summary:
http://www.wsj.com/articles/SB100014241278873241104045786258...