I think you're confusing two very different concepts - the fact the browser is sandboxed is orthogonal to its "openness." Sandboxes need not be open, and open p

I think you're confusing two very different concepts - the fact the browser is sandboxed is orthogonal to its "openness." Sandboxes need not be open, and open platforms need not be sandboxed. Fundamentally, sandboxes restrict the possible functionality of executable code to prevent it from violating some preconditions within the context a user invokes it. Rarely do developers desire that behavior (it makes it harder for your app to work!), and users complain about it too (when MacOS began requiring explicit permissions for many entitlements, many users complained about the constant nag screens). Browsers have historically needed to reinvent every wheel provided by operating systems to provide features developers require that can't work within the sandbox (cookies, websockets, etc) which prevented entire classes of application from existing. Browsers have enormous business incentives behind them, I'm not sure why you think that ruins the idea. Google spends gobs of money to prevent anyone from developing a competing non-Google browser, for example. Apple bans any non-Apple browser from existence on iOS and iPadOS.