It's the browser that allows 20 scripts from different domains all come together in the same execution context each with full permission to do anything.
No, it's not, because well-formed, valid Javascript code is permitted to depend on this execution model.
The decision to combine 20 <script> sources into the same execution context is the BROWSERS decision. NOT javascripts.
It's the BROWSER that doesn't have any concept of allowing <script> fine grained permissions within the execution context, or specifying you'd prefer if the browser put that script in a separate execution context.
It's the BROWSER that needs fixing here, not js.