You will not be able to transact any Bitcoin with help of Mastercard. They will let you buy imaginary Bitcoins on your master card account. You will not able to withdraw them or deposit. All you will be able to do is speculate about their value.
Is it? I am seeing on few exchanges are also doing that. For example in my local exchange, I cannot deposit or withdraw dogecoins but I'm able to trade dogecoins for USDT. They are saying some reason why they have not enabled the deposit and withdraw functionality for dogecoins. I am just hearing about same thing with mastercard as well. Anyhow, we can expect MasterCard to add deposit and withdraw functionality for bitcoins in near future so that people who will able to buy/sell bitcoins more conveniently.
This reminds me of the PayPal situation! They are doing something similar...
In any case, and from any side, those third-party services will have to change the ways they operate if they wish to be ready for the future that's coming! And in that future, there will not be a need for some third-party services that have only one purpose, to transact money, we are able to do it now without them already! They are becoming obsolete!
Well, all third-party services that earn from the transaction fees (a lot) to move money from one place to another are big companies, with huge capital! And I guess when you have a lot of money you can easily transform your company and start offering more popular services...