What if PageRank didn’t exist?
inperc.com
inperc.com
Most of the time people will follow links. But they'll also close pages and open new ones from bookmarks, search engines or memory.
As such, the fact that the PageRank graph is not completely connected doesn't matter, because it's already a subset of the "true" page-to-page graph anyway.
The regularization isn't just a hack to make the numbers work, it also models the uncertainty in user behavior.