6m is a great job, while it has been thrown in with some bundles it has not been thrown in with every bundle and buyers tend to have a choice of whether they want it bundled or something else.
It is also a game that sells consoles even if it wasn't as good as 2
You would imagine that Naughty Dog is 90% working on Uncharted 4 for PS4 and only a small team working on another Last of Us game (or hopefully something else). They are a 2 game studio rather than a 2 team studio, I feel there will a lot of mixing and matching.
They have also probably had some people dedicated to DLC since they have been bringing out stuff for the Uncharted games for a while
Anyway, I very much look forward to seeing what they are working on for PS4