As is the case with most complex problems like this, the correct answer is: it depends.

In this case, it depends on what the crux of your business is. Sometimes the crux is building world-class technology. Sometimes the crux is customer acquisition.

If the crux of your business is customer acquisition, then an exceptional business co-founder will actually be the most important ingredient to long-term success.

This is one of the biggest weaknesses I've noticed in YC's mantra. In most industries, just building something people want doesn't lead to success - you have to excel at customer acquisition also. And, in these industries, as you business matures, you realize that customer acquisition, at scale, is actually the hardest problem to solve.