Almost certain the thread has simply corroded between the aerial and the base over a few years. No amount of pentrating oil will get to it the way it's made, so as posted above be prepared for it to just snap off. Then you'll need a new base onto which to mount the stubby, so do it when you're happy to have this extra job.
Personally I would cut the old aerial off above the join and then get access to the fixing with grips and undo it that way, but you'll need tools and care of the cars paint, panel, etc....