In modern times, we go out of our way to make sure that we never find out that someone is a mamzer. For example, courts in Israel are very reluctant to order paternity tests for children for fear of it being discovered. As a result of all of this--and the unfortunate prevalence of "cheating"--wouldn't we expect that many of the people who we treat as non-mamzerim actually are mamzerim?
There are also situations where children are born who are almost certainly mamzerim, but we bend over backwards to find a possible way that they are not (e.g. the husband returned home from Europe over night and had relations with the wife with neither of them remembering).
When we do things like this, don't we just facilitate the spreading of mamzeirus throughout the Jewish community? Wouldn't we expect that eventually, all Jews would be mamzerim chas v'shalom?