Spending it is effectively the same thing as converting it to USD.
Why are you hesitant to convert it to USD to buy something, yet you'd be happy to have a 3rd party service handle the BTC-USD conversion in the background and collect their profit on the exchange?
There's no free lunch. Mastercard isn't doing this out of a desire to offer free services to the people. They're collecting profit by offering worse exchange rates than you can get on an exchange by yourself. Same story for PayPal and other providers.
I don't want a world where I have to transfer the BTC to USD to use it, I want a world where I can just use it. MasterCard acting an an intermediary is fine with me, somebody has to bootstrap this.
https://www.casebitcoin.com/images/stories/charliebilello_re...
I'm not inclined to transfer it to USD, but I'd happily spend some of it were it easy to do so. Maybe buy some stocks, upgrade my PC, or get a PS5 and some games? Sure, why not!
I'm only sitting on it because it's a hassle to use it, not because I believe it's going to suddenly grow 100x in size. If I really cared about the growth I'd immediately move it to some index funds.