I think once retailers (with 300-1000% markup) have a good understanding of BTC, and it's been proven it can handle the load (exponential increase of adoption) they will begin to offer significant discounts to their customers that use BTC so they can both sell off a portion for their day to day expenses plus taking an immediate profit in fiat and HODL a percentage of that markup in BTC for future risk/profit.

Any broker service that they (retailers) can trust to successfully manage their BTC holdings (for a fee) and realize a minimum 12% average interest on those holdings would simply be using sound business practice over the long run.