It was never confirmed (as far as I know) that you would be able to keep the same character for the whole '10 year plan' of Destiny (though they did apparently confirm to IGN that "progress" would carry over), but this quote from Bungie's community manager David Dague back in 2014 made me hopeful that was the plan:
"The idea is that the Guardian you have created is something you can bring along with you on that adventure. If you take a look at the way people have played other games for a long period of time, they've had a relationship with the same character for a very long time."
Source
Kind of a bummer if they changed their mind (for me at least) as I think some of the lasting appeal of games like this is to get to the end of a many year journey, look back and realize 'that's been my character through it all'. It's one of my favorite aspects of a series like Mass Effect, and going through 3 games as the same Commander Shepard.
I understand if they don't want to be restricted by older weapons, classes, and mechanics that they don't want to bring into the sequel, but I honestly wouldn't mind just bringing in my character only, and having them manufacture whatever plot excuse they need to contrive to wipe the rest of the slate clean (i.e. each Mass Effect game would let you choose your class and weapons from scratch after character import).
Though having said that, it also does also make me wonder with how much enthusiasm I'll play through the rest of Rise of Iron repeatable content, knowing I may very well be losing this character next year anyways. I suppose the simple answer is 'when it stops being fun', which is how it should be anyways.
But I suppose if that's what really ends up happening, I hope this at least paves the way for a much more robust character creator in Destiny 2.