A bug is not malware (i.e. software intentionally written to do harm). It might get exploited, but that is an entirely different story and usually requires another piece of actual malware to work.

Adding a sandbox to mitigate against bugs might sound like a good idea at first, but in reality it adds bloat and makes people stop actually fixing bugs. And a lot of the impacts of bugs can be more easily mitigated against with general system improvements that actually make the system overall better instead of adding layers of bloat. Take Rust for example: it is a memory safe language that actually improves the system, but of course a sandbox can also mitigate against software written in memory unsafe languages. But hopefully you agree that Rust is the better idea then wrapping everything in a sandbox.

As for Javascript on the web. No it would be a much poorer system overall, with people reverting to shoddy apps to supplement webcontent. Maybe you are having rose tinted glasses or are too young to remember, but the web was not better before widespread javascript use. In fact it had stuff like Shockwave and Flash to do some of what Javascript does today shudder