It's not at all clear that it was a mistake. Go has had eight years of being one of the more successful languages. If they had tried implementing generics right away, it may have taken longer and it's unclear whether Google would have supported the work for so long? Maybe the timing would have been different enough that they didn't succeed, or the lack of experience would have resulted in a design that's worse?
It's also not at all clear that Go will have similar problems as Python did, since the Go team considers backward compatibility important and might approach the problem differently, learning from Python what to avoid.
Counterfactual reasoning is difficult. Learning from history isn't as easy as some make it look.
It was successful due to massive amount of money spent by Google.
Because there were languages that are much better suited for what Go was designed for. To name a few: Erlang (concurrency that is better than Go's, "channels" and types on par with Go with Dialyzer), Haskell (thread concurrency better than Erlang's, channels, transaction memory, etc and types well beyond Go) and many others.
It's also not at all clear that Go will have similar problems as Python did, since the Go team considers backward compatibility important and might approach the problem differently, learning from Python what to avoid.
Counterfactual reasoning is difficult. Learning from history isn't as easy as some make it look.