Simple. Some other game would take the "main RPG" slot, and would become rather popular. Then, it'd get a lot of word of mouth, as people who always thought D&D was too "nerdy" were suddenly interested in this brand new idea of "Role-playing Games". People would talk about this new game, like it was something crazy-new, and it'd soon find a home in many houses throughout the world.
Even the cool kids, who always throught "dungeons and dragons" was only for nerds, would adopt this super-cool game. After all, it has a shiny cover.
And then some enlightened spiritualists would see the devil in that shiny cover, and go on a witchhunt. The media would pick up on the story, and there would be some "oh, won't someone think of the CHILDREN!?" moments. Then, some guy would randomly die in some random accident, and this new shiny-covered RPG would be to blame. There would be a huge public backlash, and players of this new game would have to defend themselves and their hobby - at work, at school, and at home.
Then, someone would discover the old copies of D&D, and start suggesting we play that as an alternative to this crazy new RPG. Shiny covers just can't be trusted.