Who cares about cores per rack
The interesting metric is cores per router interface. It is the old dispute of software vs silicon.
There are examples (and staunch protagonists) of both architectures out there. So far the silicon crowd has been considerably more successful. This evens up the odds quite a bit because you can do a 20000+ customer BRAS 10GigE or 40GigE interface card with this and simple DDR bought from the "corner shop".
That is by an order of magnitude cheaper than doing it on dedicated silicon and special router memory which costs an arm, a leg and a prosthetic.
The access (that is where you need flexibility) and enterprise router market just got way more interesting...