If this was done in the USA, a lot of airlines would struggle or even collapse if they couldn't figure out how to adapt.
The four biggest airlines in the US (United, Delta, American and Southwest) all lose money on flights. The way they make a profit is through their co-branded credit cards. The banks pay the airlines to purchase miles from them to use as points, and one of the primary ways the bank makes the money to do that is from interest payments.
I'm not saying that interest rates shouldn't be limited, just that there'd be some major impact since a lot of the financial industry is funded by interest payments.