If you are talking about crypto Debit cards, then they already exist and there are plenty of options out there. I think they are quite interesting for the next 2 or 3 years, or at least until the scalability issues are not solved. They provide an easy way to spend your coins by using the already well established Visa and MasterCard platform.

If you are really talking about Credit cards, then I don't think this is a good idea because of bitcoin volatility. You take a loan now, and the BTC price rises, and you will be losing money. If the loans are made in BTC but you only have to pay the equivalent amount in fiat, then it's the credit company that will lose money because they will receive less BTC than what they initially offered. BTC price can of course decrease, and it's the other way around, but in the end, someone will lose money, so I don't think loans with BTC are a good idea right now, so a Credit card is not a good idea as well.