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As long as you are subscribed to Stadia, you can buy a game outright, in the sense that, in theory at least (who knows what will actually happen), whenever Google decides for whatever reason to get rid of it, you still have it on your account.
This is in contrast to Game Pass, where I believe the most you'd get is a discount on the full game, but if the game is gone from Game Pass and you haven't bought it individually, your save game is useless because there's no game to play it on.