I don't think you factored in the fact that he is Canadian , he absolutely can play for 5 more but he may choice not to. I don't know if I would make a move for him period but if it was barg for Nash it would probably give me alot of pause to the point where I may just pull that trigger. I am not BC so I don't envy the rebuilding process.
This isn't hockey buddy, and the position he plays in requires the most stamina of all. Veteran Bigs generally play at their highest level for a good period of time, Guards became useless as soon as they turn 35. In Nash's case, he keeps his diet real tight and takes care of his body.