It's a little more complicated than that, but it's not that hard to define success.
On the other hand, there are plenty of nonprofit organizations around that are conspicuous for achievements such as:
- Maintaining longevity
- Raising a lot of money, or managing an endowment
- Accepting awards
- Garnering a distinguished reputation
- Arranging photo opportunities with heads of state
If they help a nonprofit organization achieve desired programmatic outcomes, that's great. If the desired programmatic outcomes are closely related to an explicitly stated organizational mission, that's really great. But if the organization is just sticking around, racking up donations, and basking in the glow of widespread approbation, then it's just going through the motions.
Over time, most for-profit organizations that can't make money have to shut down. However, there are plenty of nonprofits that continue to operate without making any significant progress in the department of outcomes. They often mask this by pointing to all of their activities, but, as they say in the evaluation biz, outputs are not the same as outcomes.
If your mission is to reduce teen pregnancy, then it doesn't matter how much street outreach, peer counseling, home visiting, or curriculum development you do - if you don't reduce teen pregnancy. If you fill a sports arena with 10,000 teens and they cheer wildly while you deliver your message, but you don't reduce teen pregnancy, then you are confusing motion with progress.
There doesn't seem to be an economic principle that causes nonprofit organizations to fold if they don't make any progress toward their missions. I don't know whether the nonprofit sector is a zero-sum game; if it is, then these organizations are simply using up resources and occupying niches that would otherwise be taken by more effective groups.