I'm all for less bloat, and I can't figure why would a browser double as a PDF reader, for instance, when a native app is invariably faster, more feature-rich, more customisable and more secure. However, it's difficult to draw a concrete line between plain browsing and web apps.
A native app is less secure. They're all written in memory-unsafe languages, are not guaranteed to be up-to-date, and do not run sandboxed. Integrating a JS PDF viewer into the browser hurts performance, but it's more convenient (no separate app to open, can start reading before it finishes downloading), and much less likely to be a security risk.
So how can we even trust the browser if native apps are always less secure according to you?
The exploit ran despite the sandbox if I understood it right.