If you only accept one form of payment then you will decrease your potential market.
See the private internet access VPN that bitcointalk advertises, they accept "mainstream" payment methods but also accept BTC and this is the selling point for the BTC community.
yes if you only accept bitcoin then your potential will be decreased because there are less people know properly about bitcoins
Not only that but not everyone has access to bitcoin when they wish to purchase something.
If someone were to adapt to using bitcoin but did not deal primarily in bitcoin they may wish to purchase this company's product but have no way of paying.