In a way, that's actually better for the longevity of the unit....as long as you changed the paste at some point. Your unit isn't going through heat/cold cycles which would cause the solder joints to crack.
there has been so many Revisions of the ps3 Slim that this is not an issue at all anymore I know for a fact there is over six revisions of the Ps3 slim model all made in just the year 2011 and one revision this year already .
each time a revision is made they tweak what needs fixing / shrinking cpu dye / gpu / motherboards heat pipes and cooling fan a fan speeds lower voltage ram power supp ect..
the list goes on
i can tell you my ps3 was made in December 2011 and after 4 hours of non stop BF3 online matches the back of my ps3 is barely warm.
the old ps3's still work
the new ones will last longer because of the changes made that the public does not even know about .
That said, it's only use is a bedroom BR player these days.
it will depend on the user
since it could be 1 day (drop it on tile floor?) or dog urinated on it? who knows..
or it can last for 10+ years..
my psone is still working even to this day.. just tested it a few months ago.. but i don't use it anyway.. mostly staying in storage..