Not gonna lie... I bit the bullet for the same reason. GS is OOS online and no stores near me have it. I had mixed feelings about this remake from the bit I played but figured $15 after coupon is decent enough.
Nowadays for today's games, I don't buy for rarity. I buy to play the game. This is be because today practically every game is patched so nothing is actually really, totally on disc. Complete editions come close but even those still often need a patch or two to fix bugs.
Rare games - i.e. games that may still be worth something in the future - mostly ended after the last generation.
Modern commercial games can't be rare when they're all available to "buy" online for a fraction of the cost.
Games like Cannon Spike, Project Justice, or Haunting Ground - games you can't readily play/access otherwise - those are likely to hold value.