What you fail to articulate is how anybody benefits from seeing the old URL.
Ripping away the original context unexpectedly is the problem. So making the original context more accessible is one good solution to this problem.
The people on the thread, being adults, write a couple sentences about how the story there were commenting on used to say something else.
For many of us, this infinitesimal inconvenience is more than offset by the improved experience of not clicking through blogspam to get to the right sources. Nine times out of ten, when this happens, the original submission is awful compared to the one we end up with.