Good code doesn't rot. I have personally written applications that lived on for more than a decade, and at least one of them (that I know of) is still alive and being used today, on a daily basis.
However, that doesn't mean one should use code they can't understand - with that I totally agree - You should understand the code you're using, even if it's copied from online resources.
If a piece of code is not self explanatory, a comment that explains it can save many hours of frustration later on.
To illustrate my last point - one more personal story: A couple of years ago I had to re-implement an algorithm I've written about 8 years before - and the fact that it was heavily commented spared me well over two days of work.
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