I think that's a common and lazy response to many security issues. There are _many_ ways in which a nefarious script or program can run in a "secure" environment and wreak havoc. Think NodeJS or Python scripts, which are typically downloaded from untrusted sources and ran blindly by most people as their own (hopefully) unpriviliged user.
> There are many ways it could steal your passwords, since desktop OSes don't sandbox apps like mobile OSes do.
Well, sure, but isn't securing this one major IMO attack vector an improvement over not doing anything about it? I don't follow this defeatist logic of "well, if you're already running malicious software, you're SoL".
Besides, this clipboard issue is also a problem on mobile OSs, since all apps share a global clipboard. Unless some app-specific workarounds are implemented, as mentioned elsewhere in the thread.