Eh, they are most likely to stay profitable anyway. There are always people who buy things spontaneously, others who don't have the credit card/means to buy online, others who are impatient and want the miniatures NOW, and even more who probably don't even visit forums. Also remember in the case of GW stores (my local ones) they are partially funded and paid for by GW head office, which turns a profit whether I buy from the GW store or from an indy.
Now independant stores are a bit different and less likely to survive if everyone starts buying online, but if thats where you play most of them give GW products for a discount anyway and get profit from other things besides GW.
Also, for me at least, it is a bit more than a small difference in price. So much so that for most things I am simply not willing to pay GW price, so they either get my business through online stores or don't get it at all. For example, it costs $50 Aus for a regiment box, the GW UK price is 18 GBP which works out to around $35 Aus. Just from GW prices alone (not online store discounts) I am already paying a whopping 30% more by buying from my local GW! If it was 5%, I wouldn't care, hell if it was 10% I would probably cop it most of the time, but 30%? Talk about being boned. I know Canada and New Zealand get it pretty bad as well.
Now if I look at my indy, Maelstrom, the price becomes cheaper again, down to $30.50 and almost 40% cheaper than my local GW prices. 40!!! Can you justify paying 40% more for a product than you have to to 'keep your local store profitable?' I sure as hell can't. Add to this that Maelstrom provides what GW never has in the form of friendly customer service and customer loyalty discounts, plus I never pay any postage.... Well, if my local store gets shut down because of a loss of business it is their own damn fault for charging too much. The market is about providing value in the eyes of the customer and clearly at the moment GW stores fail horribly. I only hope they balance it up by improving GW stores and not by screwing over indies.