If 6e is announced in January of next year, 5e will have lasted as long as 4e. And if 6e isn't released in 2019, 5e will have lasted longer than 3.5e. Both of these seem very unlikely.
But it seems super unlikely that they'll resort to that. Not without a revised PHB or attempt to tweak the product lines.
The big goal is making it past 2021, to surpass 3e as a whole in longevity.
5e seems to be doing very well on Amazon and in game stores. D&D seems to be getting a lot of visibility online. A surprising amount really. It's pretty easy to say 5e is not just doing well, but is actually a hit. It's not just maintaining an acceptable number of sales or selling well compared to RPG books, but doing well compared to books in general.
It makes sense that Meals would admit that they might do a 6th Edition eventually. That's just being honest. He's not going to lie and say "No 6e ever. This is it." No one believe Bill Slavicsek when he said Essentials would be an evergreen edition that would always be in print (didn't even make it a year).
But people aren't ready for it yet. Too many people are still playing and enjoying the game. Few people have exhausted the edition.
Personally, I'm still on my first real campaign with a couple mini games and play test games under my belt. I can do a couple more mini games and a couple more campaigns before I'll have played as much 5e as I did Pathfinder. And 5e is a less broken game. That's not counting breaks to try other systems: always a good idea. Looking forward to some FFG Star Wars. Maybe some Vampire. I could play 5e until 2020 without issue.
Though I do hold out hope for "D&D tactics" (i.e. 4.5). Which Merls also mentioned. I'll guess 2-3 years for that.
Maybe do that with WizKids or GaleForce9 as a boxed set with minis. Include power cards and the like.