Seriously. I have a featureful PDF viewer I am intimately familiar with. I want it to be the default for all PDFs, ever. This gimped viewer in the browser is not what I want.
Also, it can be useful to keep the PDF in the context of the app you opened it in. Unfortunately, I'm not aware of a window manager that surfaces the fact that, for example, one macOS Preview window was opened from the browser, another from Slack, another from Finder, etc. Compare to iOS, where opening a PDF viewer from an app will result in a button at the top-left corner to go back to the app you opened it from.
The previous comment was not talking about the browser viewer, it was talking about various website viewers, like the one by Jira.
I agree website viewers are pointless. But most of the time I actually like the browser viewer better, if it opens links directly, than offline viewers. Because I regard PDFs as websites (similar to jpeg files), and I normally don't want to accumulate them in my download folder.
I agree though that the browser viewers are often too bare-bones.
Which tend to be, imo, even worse! I think I'd rather have a toner container explode on me than try to suffer through the experience of using Aconex's PDF utility ever again.