I can see Durant losing popularity due to Heatmor, but I don't think it'll completely wipe the deck off the map. Heatmor is great against Durant, no doubt about it, but imagine that you're building a deck and are deciding on the last few cards to include. Would you rather play a card that could potentially help out against multiple matchups, or a single matchup that's predicted to see less play anyway? Unless your deck can already win against everything but Durant, I would expect most people to pick something else.
Durant won't see as much play, but I imagine the new Durant decks will be designed with Heatmor in mind, possibly including more basic pokemon to reduce the risk of donks and pokemon that can counter Heatmor. I wouldn't be surprised to see a card like Bouffalant make it's way into surviving Durant decks (Search for it and a DCE with Twins and boom, no more Heatmor for a while).