This is unfortunate; it would be nice if dissertations were the beginning of a "forever" project, or perhaps the continuation of one. Sadly, most of the dissertations I have seen have been a bunch of quick projects stapled together with one overarching theme, with a minority being answers to big questions (like Craig Gentry's thesis on FHE). I can think of only one, in fact, that was the beginning of someone's "forever" project, and that is Darse Billing's dissertation on computer poker players (he seems to be spending year after year working on poker AI).
My experience in grad school has been this: we are discouraged from having "forever" projects, and encouraged to have "lots of little projects that you finish and forget about" dissertations.
I very much see dissertations as the beginning of forever projects; but maybe that's because my advisor's dissertation was†, and his own advisor (Ishii, cf my first post) saw it that way as well.
Which is what led me to quit my PhD (only leaving grad school with a measly master's :) )- I felt that I was too young and inexperienced to embark on such a journey. I fully intend on going back to grad school in a couple years' time, and treat my PhD as such then.
