You're drastically overestimating the storage requirements. Time is far, far more valuable than storage savings. The point about CI builds running faster is enough to sell the idea all by itself.

Oddly not mentioned in the article is that it allows you to build when npmjs.org is down or unreachable, which happens often enough to be frustrating, and if it happens when you're trying to deal with an emergency, it's downright infuriating.