I don't assume that applications are inherently hostile, but I do not trust that the developer of every application I use to open untrusted input is sufficiently security conscious to prevent those applications from being subverted.

I mean I trust Firefox to not steal my data, or maybe $APP_NAME, but do I trust the C++ code? Or maybe it's an app that's been pwned a lot.

Sandboxing has uses other than running actively hostile apps (for which I'd argue this kind of sandboxing isn't enough).

No, I absolutely do not trust the applications I run natively. I trust the hardware, the OS, and the sandboxing layer. Mostly because I have to. I absolutely do not trust Spotify or Steam with my personal documents

I generally trust the applications that I run natively to not be malicious, but I don't always trust them to not step on each other's toes, or make a well-intentioned mess of things.

A “good” pdf reader only needs a bug and a bad pdf file.