At least I knew this when I started. And it motivated me to get finished in 3 years.
Someone did give me the advice in this article before I started. Which was good. I wasn't in it for the money. It was the freedom to work on things that interested me that mattered.
What no-one told me later, when I started a post-doc, was that the chances of getting a full-time academic job were close to zero... now that did feel like 3 years wasted.