It's the safe, predictable thing to do, which I suppose is the appropriate route if they're going to make a sequel. If I had to pick between one or the other, I guess I'd have gone for new protagonists. I didn't particularly care about Joel, and Ellie was mostly just kind of there. I guess I can see this as the series' second chance to build on the characters.
My trope alert goes off because of the time skip though. With both being adults now, I'm suspecting one of two routes they'll take this. The first thing that came to mind, was that Joel is going to bite the dust. It's kind of a thing in this genre of fiction, give easy drama, and it would give purpose to bringing them back. The other possibility is that they could turn it into a co-op experience with two adult characters, though I'm not sure how compatible this is with stealth and the cinematic flair. I would also assume they'd make a point to advertise a feature change like this more prominently too.
As a side-prediction, the laws of game tropes state that if Ellie is indeed the main character, she'll have to get a bow at some point. It's her white brunette destiny.