Well, you can have a successful product with a better or worse code-base. Of course, if the product is successful, the code has to be good in some sense. But that doesn't mean that there aren't parts of it that are in bad shape.

For example, I've worked on a huge C# product that has been developed in various epochs by up to 100 people at a time. When I was working on it, it was around 10 years old. Certain parts of it were in good shape, others had stuff like a 5k+ line OnPaint() method for a table cell that was doing data processing in place and had stuff like "if ((value == -2) && (type == someTableType) && (userOptions.SomeFlag == SomeEnum) && (someOtherColumn.Value == "something")) { value = paintCheckBox(enabled) }" and such.

Of course, that code was doing what it was supposed to (at least in general), and it had gotten to this state during a really bad delay where bug fixes/features had to be churned out to reach an important release date - so in some sense it was "good". But in the sense of being possible to change without a major re-write (of that particular area I mean) or without introducing 3 bugs for obscure combinations of features, it was really bad code.