Christianity holds that Jesus is unique in being the incarnation of God; there is no other occasion on which God himself became fully human, lived, died, and rose again in order to save us. (This is not to deny that there have been other occasions before and since when God has revealed himself to humans in some way.) This is clear from the New Testament witness, which uniformly presents Jesus as the one, not one of many. It is also consistent with the historical understanding of the church, as expressed in the great creeds and the surviving writings of significant early figures.
Let's suppose, however, regarding the problem of "what about those who have never heard of Jesus", that there are several historical figures who are as good as Jesus. But then we still have a problem: what about the people who've never heard of Jesus or Buddha or Lao Tzu? In order to achieve total coverage, we would have to posit a large number of divine manifestations, in every time period and every geographic area. The observed spatiotemporal distribution of Great Moral Teachers doesn't seem to match the theory. And even so, we have to answer "what about very young children?", "what about people with severe mental disabilities?", and so on.
The larger problem is clarity about just what these multiple instantiations of God are for - what are they there to do? The selection of Buddha and Lao Tzu as candidates suggests that some kind of moral instruction is the answer. However, Christian ideas about salvation are linked to the person and work of Jesus - not only his teaching, but his life, death, and resurrection. These events do not need to be repeated:
"For Christ did not enter a sanctuary made by human hands, a mere copy of the true one, but he entered into heaven itself, now to appear in the presence of God on our behalf. Nor was it to offer himself again and again, as the high priest enters the Holy Place year after year with blood that is not his own; for then he would have had to suffer again and again since the foundation of the world. But as it is, he has appeared once for all at the end of the age to remove sin by the sacrifice of himself." (Hebrews 9:24-26, NRSV)
"We know that Christ, being raised from the dead, will never die again; death no longer has dominion over him. The death he died, he died to sin, once for all; but the life he lives, he lives to God." (Romans 6:9-10, NRSV)
"Christ also suffered for sins once for all, the righteous for the unrighteous, in order to bring you to God." (1 Peter 3:18, NRSV)
Moral behaviour is part of the story, but it is not the whole story. It is certainly not sufficient, on its own, to reconcile us to God and one another. It is God who redeems us.
Many people have had the idea that even though nobody comes to God except through Jesus, this may happen even when those people do not know about Jesus. Some related answers from this site: