There's a myth going around that Darwin repented on his dead bed, something so prevalent given how utterly irrelevant the argument is. If Newton renounced gravity, it wouldn't mean that apples ceased to fall straight down. So the patterns in nature of what Darwin spoke of will still be there regardless of whether Darwin renounced them. If you shoot the messenger, the message remains.
So if it were true it would be irrelevant, that it's not true makes it all the more pernicious. But why should the argument work at all? Perhaps it's in the courage of others convictions that we find confidence. Experiments have shown that individuals are more willing to trust "experts" on the basis of confidence even if they're not very successful as predictors. Likewise, the science of evolution is not very well understood outside of scientific circles and most people are subject to authoritative statements by relevant experts. It's not hard to see that if Darwinism doesn't even have the support of Darwin, then what does it have left? And we are story-telling creatures. A story about a man who was opposed to God repenting on his death-bed might be a quite powerful one to the right audience.
For whatever reason it persists, it's important to remember that it's quite simply not relevant to the truth of evolution whether Darwin believed in it or not. The point of this exercise is making a argument from authority, and even though biology has moved on since Darwin he's still the great scientific authority.