It may be the only application that a user uses with a given library. It may be generic to the programmer, but not necessarily from the browser's perspective.
Uh, buh? That can be the case with any library. You can say the same thing about SCGUI if you're only running a single app with it. WTF kind of a stupid argument is that??
A generic library is a generic library. It gets downloaded once and cached. You're reaching.
1. Complicate the browser design
Are already coded, complete, and installed on 90+% of the desktops out there. The complexity has already been paid. Insufficient argument.
2. Increase versioning problems
In what way? As opposed to SCGUI where the user has to download a completely new version of the browser when new functionality comes out or is needed, in stark contrast to downloadable JS libraries which change immediately on the server with the app that includes them? SCGUI has the versioning problems here.
3. Create security risks
On a secure VPN application used via an extranet or intranet connection? In what way?
4. Increase download times
Wrong. The JS is downloaded once in a library and cached.
5. More things to crash
What? How so? How are there "more things to crash" than any other client-server architecture? You're reaching again.