Can it really be zero? I mean, the average startup might not manage to break even. But the insight, experience, and knowledge gained should not be understated. Plus startups have to buy hosting, equipment, domains, etc. They do not generate profits, but they do help keep other businesses running. By doing so, they do contribute towards meaningful impact. It might not be world changing, but might be life improving for a small set of people. Reason that the people who get paid from the money paid by startups to cover operation costs is used to keep a roof over their head, put food on the table, send their children to school, etc.

I could also argue that failed startups help others by weeding out businesses plans/ideas that might not exactly work (at a given moment in time/space). Look at the many failed startups from the dotcom bubble. There are plans/ideas from that era that will not approached by the current batch of startup entrepreneurs. All because they have the historical data/evidence from the past bubble.