I know people have strong feelings about this, but the reality is we made the trophies the way we did because we didn't want the game to declare what is a "collectible" versus what's just a cool thing to stumble upon in the world, or to encourage players to choose one kind of conversation over another, or to try and explore specific areas rather than others, etc., etc. We really did consider just about every possible avenue, and we ended up on the simplest avenue we could think of, not because we don't care or don't think it's important, but because after laying out all the options, we think it makes the most sense for this particular game. Firewatch is a game with a lot of player choice and hidden stuff, but because of the overall vibe, we didn't want the sensation of trophies/achievements putting meta-commentary on top. And there are a number of conditions you have to satisfy for a platinum, and we just didn't end up with one.
It may well be that we're giving up some sales by not having trophies work a certain way, but on the other hand, we think that the design of trophies/achievements is a part of the game design itself and so it's not really appropriate to try and min/max sales by way of trying to guess the ideal way to incentivize people to buy the game for trophy-related reasons. The reality is, there are infinite reasons someone might buy or not buy your game, and you can't try and chase them all down; you just try and make the best game, and hope you do a good enough job marketing it that a bunch of people will know it exists and buy it if they think it looks compelling.
For what it's worth, I would love to make the kind of game some time that really takes advantage of the sorts of overlapping mechanical goals that trophies and achievements are suited for. I love that kind of game. It's just not what Firewatch is.
Edit: I also have to again state that people are free to disagree with any design choice we make (although preferably that's after playing the game). I just want to make clear that whether or not we're right or wrong about any given decision, they are all made in good faith, not out of obliviousness or financial idiocy.