Why can’t you do both? If you are going to make a fiat purchase use bitcoin instead and buy more bitcoin with the fiat you would have spent. 

So if the merchant won’t accept bitcoin:

1) take the fiat you would have spent and exchange it for bitcoin

2) pay the merchant with a fiat credit card

3) when the credit card payment is due 30+ days later, exchange enough, bitcoin to cover the fiat payment

Most of the time you will have bitcoin left over from doing this transaction that you can HODL keep stacking. But even if you don’t, you have lived on a bitcoin standard as a first principle