There is nothing stopping them from sending a request to that merchant asking them to accept bitcoin. For example about 5 years ago people bombarded Valve with requests to accept bitcoin payment and they did it when they saw the demand.

As for your paradox, I used to see it that way too but no more. I believe it is a natural process that will happen slowly. When there is demand both sides will adopt bitcoin.
But the real issue is the price. When we see price keeps going up month after month or year after year, we end up with people who have bought bitcoin as an "investment" not as a currency so they are not willing to spend it in first place so there isn't really that much "demand" for merchants to accept it. Although I should add that even with this situation we are getting adoption in both camps: investors and spenders.