I've always thought they had since it was implied all through the series that the Minbari were a dying race and, in order to survive, had to combine with the other half of their souls -- humans. The birth of David Sheridan was the first step in that process.
Then again, we could say that Jeffery Sinclair/Valen having progeny was really the first step.
I don't think it was ever implied that the Minbari were a dying race and JMS did say that the Minbari do make to the end.
http://www.midwinter.com/lurk/countries/us/guide/088.html
In pre-production material, JMS and others make reference to the Minbari as a dying race, hence the cooler look of their technology and artifacts.
Moreover it's more or less stated in "Points of Departure," when Lennier reveals the reason for the Battle of the Line surrender, he makes mention to lesser birth rates and the diminishing of each generations' souls:
"Over the last two thousand years, there have been fewer Minbari born into each generation. And those who are born do not seem equal to those who came before. It is as if our greater souls have been... disappearing."
They are dying, but not in the Markab sense. Birth rates are plummeting and given the course of time, the Minbari would either be a scattered handful or no more. Moreover, their culture and society has become stagnant. They lack some spark, and it is in their contact and eventual partnership with humanity that the Minbari are able to progress again.
My interpretation is that David Sheridan, or even Sinclair/Valen's progeny, were the first steps to a new hybrid human/Minbari species. Now that doesn't negate the existence of separate human and Minbari races in that faraway future. A tri-culture of human, Minbari, and the hybrids. How perfect is that?
As the Minbari believe, three is sacred.