The more I think about this, the more I don't like any of the possible explanations. So that leads me to wonder about a few things. How sure are they that he had no children with other wives?
Well, there are only certain possibilities to start with--children of women he was sealed to, who were born in the right time period. The list in the Wikipedia article shows only 9 possibilities.
https://en.wikipedia.org/wiki/List_of_Joseph_Smith's_wives
I haven't read the source material for the Wikipedia article, so I can't say whether that list is exhaustive, but it's larger than I expected from my previous research (~15 years ago). Out of the 9 possibilities, they've apparently done DNA testing on the descendants of 5 of them, all negative for being related to the Smiths. 2 of the 9 died in infancy, so no DNA testing possible. The remaining 2 apparently still remain possibilities.
Of those two untested ones, the one I'm most familiar with is Josephine Lyon. As the Wikipedia article states, her mother told her that she was Joseph's daughter. That matches my previous research. Based on what I remember reading back 15 years ago, it seemed like a strong possibility that she really is Joseph's daughter. I don't know anything of the other one.
So, did Joseph have children with women other than Emma? I don't know. It seems possible. But it's nearly certain that he didn't have MANY children, and very possibly only one.
As much as some posters believe this was common knowledge, I know my family members who are lifetime LDS members knew nothing of this. Is it possible info may come out to the contrary? Do they have evidence that proves he had no other children, or are they assuming because there has been no evidence of children found, there must not have been any? For some reason, I have to believe if they know for sure, then they should have some idea of why no other kids.
I'm not quite sure what you're asking in that paragraph. Is it possible that they may find unknown children...is that what you're asking? I doubt they find any unknown children at this stage; it's been quite thoroughly researched. But it's possible that DNA testing on one or both of the untested possible children's descendants may turn up positive. And it's possible that one or both of the children who died in infancy were Joseph Smith's.
I'm also wondering if JS was supposed to introduce and implement, does that not mean/imply he was supposed to lead by example? Also, what about being commanded to populate the earth? You have a bunch of wives, but you choose not to procreate with them? None of this adds up.
As I remarked earlier, while I do believe Joseph Smith was married in name and in deed to some of the so-called "40 wives", it seems likely to me that the majority were simply sealings for the hereafter. From what I've read, there was a common belief in the early church that being sealed to a high church leader would enhance one's station in the afterlife. And such sealings wouldn't necessarily be part of a marriage in this life. If I recall correctly many women were even sealed to Joseph Smith after he died.
So in the end my own opinion is that yes, polygamy was instituted to "raise up seed unto God", as the quote from Jacob in the Book of Mormon goes, and was instituted by Joseph Smith for that purpose, but the effect wasn't fully realized until after his time.