I have a 2009 MBP now, but will eventually buy a Retina MBP.
So do you just stop using the MBP? Put it away and enjoy the moments you had? If you just completely stop using the computer, is that bad?
Keep it as a backup computer.
I still have a 2009 17" MBP for this reason, and have also performed some upgrades to it (8GB RAM and an old Samsung 830 SSD taken out from an old Mac Pro).
It still has quite some useful life left in it.