-With WotC's new plan for 5E being all-edition-inclusive, I see the longevity of 5E being very long. Instead of releasing a new edition in 4 years they will simply release a new "add-on". In effect, they will be releasing a new edition but calling it something else. This is brilliant from a business standpoint. They could produce new material for one edition (5E) for many years to come without disrupting the core rule set.
-One of the complaints of 2E was that there was too much diversity in settings. D&D began to compete with itself. People playing Birthright didn't buy Planescape or Dark Sun and vice versa. TSR found themselves producing material for a variety of settings with an ever-decreasing amount of customers. Overall they may still have had the same number of customers but their money was now spread out into a larger pool of products, thus decreasing the profitability of each product line.
Will 5E go the same way, but with a variety of "add-ons" instead of settings? Will the add-ons compete with each other for consumer money? For example, if they release an adventure for a simple 1E inspired add-on and another adventure for a tactical 4E inspired add-on they will be producing 2 products that may only interest one type of player. There may be enough interest to sustain both lines, but where will the breaking point between support and profitability be?