Having been at 3 companies that did this, I can say with certainty that our caching proxies (artifactory in all of them) were much more likely to go down than the public repos were.
Yeah, I think I've lost a day in total when our local npm was finicky on several occasions or failed to fetch latest version of a package that we've developed in house, but made publicly available.
They just don't have to be down at the same time to be useful.
Yes, but at least in that case you have the fallback of the public repos.