I know I'm going out on a huge limb here, but I am telling you that 5E is the last edition of D&D. This is both literal and figurative.
D&D has nowhere to go now as a system. Throughout the various editions D&D has slowly changed with the times. The first edition and the various iterations of that gave us the core concepts of role-playing games and what D&D is...including the concepts of variability and additive systems that can be attached to the core system. Future editions gave us further enhancements and refined ways of doing things. 2E gave us a more streamlined system and near its end more sub-systems. 3E took the concept of additional material/systems to the max. 4E added in system balance. D&D has been ever evolving and showing us new things with each edition.
5E stated that would be a modular system. A simple core that would allow for additional systems to be added in seamlessly. They actually gave us that (much to my surprise). 5E can take the abuse of new support material...but it also can play lite. There is no reason to release a new edition, 5E allows for new concepts to be added onto its core design. If someone has a radical idea 5E can handle it, just release a support book with the radical idea and detail how to integrate it with 5E. Whatever evolutions in game design emerge can be handled by the modularity of 5E.
Of course, some of you are thinking that there will eventually be a 6E, if only as a money grab. And that might indeed happen. However, this brings up my second point. A new edition of D&D will die. It will be a non-factor, a non-issue, a non-game system. People will ignore it as nothing but a money grab. They will see no need for a new edition. In essence, 5E is the last viable D&D edition.
D&D never needs a new edition and people won't want a new edition. Between those two points, 5E is the last edition of D&D.