Bitcoin will never be as "stable" as something like the USD — heck, even gold isn't that stable. It's just that as time goes and as more and more people use bitcoin, bitcoin wouldn't be as volatile as it is today.
Just look at what people are saying right now, experts say that people shouldn't hold USD because inflation is just 5% a year, but imagine if you have bitcoin that can go down like 5% a day because some whales sold it. It will be problematic for people to use it on a daily basis because of that.
Because you don't buy your bitcoin to immediately sell it in a day. Usage-wise, probably wait for it to appreciate by a few points before actually using it. There's a reason it's almost never recommended to go 100% on bitcoin — always leave room for your fiat currency
True, I don't really understand why people still talking about BTC becoming stable. It's not even possible. Yes, it won't be as volatile as it was for the last decade, but still, there is a big room for growth. And you can't rely only on Bitcoin anyway.