When I was in grad. school, I thought the idea of doing a PhD was to train for a lifetime of contributing to human knowledge. Not a lot of PhD theses end up as a contribution to human knowledge. Sure, there are the 1% that do, those people don't typically have the problem of not being "able to continue contributing."

Agreed. You get a PhD for you, not for science. I would hazard 99% of PhD dissertations get read cover to cover less than 5 times. And I'm including the committee and author on that number.

That is, broadly, the ideal. Your PhD thesis is your first masterwork that qualifies you to call yourself a real researcher, and you go on to do many more along the length of your career.

In actual fact these days, there simply isn't a career model set up to support that anymore. Your PhD thesis is now supposed to qualify you for your first postdoc, your first postdoc or two qualify you for a permanent position, and you earn tenure or permanent contract in your permanent position, and then you can just focus on contributing to knowledge rather than on careerism.

I am currently working on my PhD Biomedical Engineering (3 years in), and I have mixed feelings on this. Certainly most PhD dissertations are a joke, but during the first few years of a PhD you are acting as the arms and legs of your professor, who, if they are any good, is capable of pushing forward human knowledge using you as an "instrument".