Writing code always involves starting with assumptions about how the code is going to be used. Extensible code is really just code that includes in its design assumptions that a part of it might need to be switched out or accommodate something more in the future.
Sometimes its easy to predict how your code may need to be extended if you give it a bit of thought. In those cases, it might make sense to allow for easy extensibility. This is pretty rare though. Most times, you can't really predict what will be required of the code in the future. In those cases I prefer to bake as few assumptions into the code as possible, so when someone else is extending it in the future, they spend as little time understanding your code.
Sometimes its easy to predict how your code may need to be extended if you give it a bit of thought. In those cases, it might make sense to allow for easy extensibility. This is pretty rare though. Most times, you can't really predict what will be required of the code in the future. In those cases I prefer to bake as few assumptions into the code as possible, so when someone else is extending it in the future, they spend as little time understanding your code.