That's easy to answer because he's a bench player, and while a useful one, his skill set (good midrange shooter, good finisher in traffic, decent iso option on occasion, better rebounder than his size would indicate) isn't absolutely integral to what the Warriors need. It's useful, not critical.
However, given the Warriors offensive changes after David Lee's absence, if one assumes the Warriors will adopt a hybrid stance next year when Lee comes back (i.e. try to keep the offense spread out, but try to assimilate Lee into this style of play without significantly reducing his role), then Landry is definitely expendable.
Lee can do all of the things Landry does on offense, except add in much superior passing skills, better overall finishing skills inside due to the whole ambidextrous thing, and also being a superior option in the pick-and-roll due to being a better cutter.
In a sense, both players aren't really a part of what we *might* be seeing (SSS caveats notwithstanding) -- a better Warriors offense based on Draymond Green or Harrison Barnes at the 4 -- however, I think we all know that the Warriors can't/won't relegate DLee to some significantly lesser role and that he'll likely log nearly as many minutes next year as he did this year, and his contract renders him well-nigh untradeable (and would also likely be too large of a psychological/public relations pill to swallow).
In which case, Landry is, unfortunately, superfluous.
I'm sure the Warriors should still have another big on the roster to be able to come off of the bench, but as SleepyFreud mentioned in another thread, the best skill that big could have for the team as it currently stands would be to have good defensive skills (more shot blocking, please), hopefully with some athleticism thrown in the mix. Being a team that generally goes small shouldn't preclude them from having personnel that allows them to have the option to match size.
In any case, it makes me sad, because I like Landry a lot and I think he's done all that's been asked of him and come through on all counts...he'll probably continue to do so for as long as the Warriors remain in the playoffs, too. But the minutes he's getting now likely would all go to Lee next season, and there likely won't be that perceived need for a bench option to back Lee up as that would probably come in the form of Green/Barnes.
Of course, Landry has a player option for next season, so much of this depends on if he decides to exercise it or not...could be that he'll decide not to pick it up and look elsewhere. But if he exercises it, I suppose he'll be an offseason trade chip that I personally think a number of teams could be interested in.