what is the potential for the price to increase on the 79 Beetle convertible?

At the present time, not much. It seems like the cars that are appreciating are the late 1960s muscle cars, and the earlier vehicles. The late 1970s vehicles, unless of some extreme special interest, have pretty much stagnated and are viewed right now as "old cars." It has been this way for at least the last ten years. I have seen the supply of older vehicles dry up, but I can still get almost anything from the late 1970s for a song. Unless it is a pristine original or something very special, I doubt it will increase much at all. Take for example the 1977 Chevy Vega GT with 61 -- yes, 61 -- actual miles sold at Kruse auto auctions this past fall. It only sold for $13,000. Adjusted for inflation, the owner LOST almost $7,000.