I think it's hard to say whether cards are undervalued just based on how much supply there is because prices are largely driven by demand. It's just the case that fewer ToTG cards are in meta decks right now as compared to Genesis, and if future expansions introduce new meta decks with many ToTG cards included, their price will shoot up considerably.
I also actually think Shiny card forging won't affect the supply too much unless the shiny rewards turn out to be much better. Most people won't have 5 or more copies of a certain card unless they buy large amounts of packs.