If I remember properly, old lightguns basically took a picture of the screen when you pulled the trigger. Switching to the new LCD, Plasma, and other screen types in TVs was a problem mainly because those other TVs had different refresh rates, or methods of drawing the screen, resulting in the information that the gun used to track its shots being garbled in the image it took.
It's not technically impossible to get old lightgun games workin with the new guncon. However, it's not as simple as making the guncon 3 "backwards compatable". The fact is, those old lightgun games were written completely with the old lightgun tech in mind, as such, you'd have to have a programming team go in to hack apart the old gun tracking code, then hack in a method where it reads the new guncon coordinates and translates them into the game's native coordinates.
This means you'd have to pay a team to do it.
I suppose one theoretical way of handling a universal backwards compatability for old gun games is to run them with some kind of code wrapper, something like a screenshot utility where whenever the player pulls the trigger on the guncon 3, the system takes note of where the guncon is aiming, then takes a screenshot that's at the appropriate spot where the gun would be aiming, then feeds that data back to the game as if it were an older lightgun.
TMK is both right and wrong on this matter. It's not as simple as it seems, but it's still possible. Heck, it may even be required for someone to port these games if they want the guncon 3 to be successful. It's clear that they weren't willing to risk money on a full game. So the best they can do now is try to get other developers to support it.