However, SPY can't go further down than $0, so the gains aren't "unlimited". In the unlikely event of armageddon, you would get paid the maximum 300*$163 = $48,900, and it's not clear what would be left to spend it on.

Theoretically, you could buy infinitely many puts, with strike prices of $163, $163/2, $163/3, $163/4, $163/5... which have an infinite sum, but because each is exponentially less likely to end in-the-money, the prices of the puts total a finite amount. I don't know of a broker which will sell infinitely many puts, though.