The Last of Us was a game about Joel and Ellie, that's what people want more of. A game with an entirely new cast of characters is a far riskier premise than using the two who have already been created and brought to life by their actors. I imagine they'd have sooner created another IP altogether than do that.
The story of the first game is done and finished; this isn't a continuation of it and it doesn't detract from or "spoil" the ending. It's five years later, who knows what might have happened in that time to feed-in to the events that take place in Part II.
I'm not sure why you think the people who created the first game that you seemingly hold in such high regard can't do it any justice with a sequel.
I agree that I wouldn't want a story without Ellie and Joel but I don't think anyone can argue that it doesn't detract from the first game's ending. People won't think about it as a separate story simply because it features those two and expands on their life after the events of the first game.
I'm absolutely positive we'll end up with an outstanding game in many regards but I really don't think they'll be able to deliver an equally compelling storyline. Think about it: after such a beautiful, balanced journey with a very powerful ending they can easily fall into clichés or go all melodramatic because they'll be trying to top it. The Last of Us was unique partially because people weren't expecting such a story and such and ending. Naughty Dog had more to work with.
Now people have sky-high expectations and are already forming theories about the story:
-Fireflies hunt them down
-Ellie finds out about Joel's lie
-they eventually part ways and become enemies
-Ellie kills Joel in the end
-Tommy finds out and sells them out to the Fireflies
-they both die
-Joel dies along the way
-Ellie finds out something bad about her mother
-ND go with yet another "gray ending" that simply won't have the same impact
-ND do a completely separate story not in any way connected to the first game which will seem odd and will undermine the ending even more;p
I can easily understand why ND said that Ellie and Joel's story was complete and that they were struggling with inventing a worthwile scenario for the sequel.
I'm still positive but very curious as to how they're going to avoid clichés like the above and deliver something equally impactul and not predictable with the bar set that high.