If D&D tanks you would see the collapse of the current fad, and this would lead to an important percentage of exclusive 5e players just quitting, and others just continuing to play the older edition in their own groups.
A certain percentage would move to other games, particularly if we saw a Pathfinder-style scenario, where a direct rival appeared made to appeal to 5e gamers (and with enough corporate power to promote it enough).
Some would migrate to other existing games, some would look at the OSR.
But of course, the current mass-popularity of D&D would collapse.
The hobby itself would continue. And if you later saw a new edition that corrected the insanity (like that 5e was for 4e) you might see another revival. It would probably take about a decade though.