The continued viability of D&D as a hobby is predicated on a more or less continuous influx of new players. Some people get inmto the hobby and stay in it for the "long haul". I've been playing RPGs in some form or another since the 1970s for example.
However, guys like me are in the minority. Most gaming groups have at least a couple of guys who got into the hobby within the last few years. People leave the hobby on a regular basis, as they become disinterested in playing anymore, their spouses pressure them into stopping and the commitments of work and family pull them away. We need the new players to replace the losses.
Most new players are brought into the gaming world via word of mouth, and FLGS's. Most people buy their first books from an FLGS, many people who now game would not game without that FLGS. Without the FLGS, we get a significantly reduced influx of new gamers into the hobby. Players become fewer and far between. The pool of people willing and able to form a game group becomes thinner and thinner, until there are only a handful of isolated people who may or may not be gaming.
The market for the product has shrunk. The borderline profitability of game products is now damaged to such an extent that products are no longer profitable to publish. New material becomes nonexistent. You still have your game books, but without materials available, you have very few players to join you. The books sit on your shelf, gathering dust, because you have no one to game with.
