Right. A good debt is a debt that you can afford to pay. Investing your borrowed money in bitcoin is a bad debt because you wont know when youll be able to get the money back. Personally, i borrow money or do installments only for the purpose of maximizing the time value of money. When it comes to cryptocurrency, you should only risk money that you can afford to lose and I dont think that counts borrowed money.
But still, it isn't a bad debt because you are investing it, meaning you will profit from it if you invest it at the right time. It is not like it's our wants that we don't need it; we call it investments or business, just like trading; the problem is just the risk, so it is still including to good debts, but it doesn't mean that you'll take a loan now because it is a sure profit; you should also make sure that it is a calculated risk that you can repay your loan.
There's still no guarantee you will profit in a volatile system of Bitcoin. Even if you invest at a right time but when it changes its value to low then you will have a problem in your debt. That's why its considered as a bad debt, you can't rely your money if its borrowed to a "risk" its like you're saying that invest your debt to a luck. If you can pay it sooner or later, then borrowing money doesn't necessary if you have spare money for investing. He's right its common phrase when trading "invest what you can afford to lose". Maybe it might work to people who's already have lot of experiences and knowledges in loan and trading where he can simply use it to gain money, but if he's really good then he doesn't need to loan since he already have profit to invest. Therefore its still considered as bad debt with the risk that it involved in the situation.