You might perhaps have a setup where trusted users are running trusted code, can guarantee* a lack of malicious intent, and primarily want the user-and-process-segregation system to prevent a single failure of a single process from affecting anyone else.

You might conceivably have this in a corporate environment (I do).

For the vast majority of systems that do run untrusted code, including web browsers, this mitigation is probably appropriate.

Perhaps the word “might” here could be upgraded to “probably”?

*for some arbitrary value of “guarantee”