5e/5.5e will probably see around a 50% split initially. But if it's as seamlessly backwards compatible as WotC claims, the 5e players will continue to purchase 5.5e content at the usual rate (to get the 5.5e equivalents of Styxhaven, Fitzbong's Dragons, Critical Rule Campaign Guide, etc.)
But then...
Moving into the "D&D Brand" mentality will be the bigger challenge for fans. I can see D&D becoming a brand like "MTV." One day we might say, remember when D&D was a roleplaying game. It's going to be all the stuff around the hobby that makes the money, and we'll be nostalgic for the day when D&D used to release adventures and rules supplements.
But then...
Moving into the "D&D Brand" mentality will be the bigger challenge for fans. I can see D&D becoming a brand like "MTV." One day we might say, remember when D&D was a roleplaying game. It's going to be all the stuff around the hobby that makes the money, and we'll be nostalgic for the day when D&D used to release adventures and rules supplements.