Replies from others are emphasizing the use of BTC for gambling, but the keyword in my subject title is "reserve".

My point is that BTC is the unit-of-account by which everyone measures their gambling success, not fiat.

When someone gambles on an altcoin "investment" (speculation), they are hoping to get more BTC. They don't cash it out to fiat, they HODLit to gamble some more altcoin "investments". Even if you include gambling sites that accept Bitcoin, the gambler is likely HODLing their BTC gains (if any) and not cashing out to fiat.