If we're talking about a sequel that goes away from Joel+Ellie, I'd kind of agree, but if we're talking a direct sequel to those events, it's not the same. As hard as you try to not let it affect you, expanding on those events through a sequel will absolutely color the way you view the end of TLoU, and will take away from what makes it so good. The point of that ending is the discussion found in this thread and the spoiler threads. If they wanted to spell everything out, they would've. Instead, they clearly back away from over-explaining past events and character emotions several times through the course of the game, and coming up with some contrived sequel or further fleshing out these characters will inevitably disappoint, something Neil and Bruce have spoken on concerning the Ish letters.
Leaving that story as it is transforms it over time into something more than it would be by watering it down with sequels. That applies to the game as a whole as well, which is why I still think The Last of Us would be better as a one-off. Over time it will build prestige and stand as a work that they respected enough to let breathe, especially since it doesn't feel like it needs a sequel. EVEN THEN, I'm not saying they can't reuse the gameplay, or the tone, or themes, or even the universe they've created in some kind of sequel, but if they do go that route, at the very least I'd like to see Joel and Ellie's story remain untouched.