Yes, this might make securing your device easier. But more and more, what I value is on the network - not my local hard drive. Be it a local network git reposit

Yes, this might make securing your device easier. But more and more, what I value is on the network - not my local hard drive. Be it a local network git repository, my ISP's email system or my cloud based apps (including my bank's internet portal). So this hypothetical browser.exe with "only" network connectivity still needs just good a security model as today's browser. And how do know the new browser.exe I downloaded today doesn't have a poisoned security model that allows it to sniff and send my internet banking credentials to the bad guys?