BD made an excellent point, and I didn't see your response. Paul says flat out that he had finished his course, that his work was done. Since Paul was so dedicated to praying for others, why would he say something like this if he had any thought of continuing prayer for others in Heaven? Paul gave words of encouragement to others all the time. It would seem automatic, to me, that he would comfort his disciples, especially in writing, with the knowledge that he would always be praying for them, even after he is gone. Yet, Paul appears to snub them all. That is, IF he believed as you do.
Another thing, in order for a saint to know how to intercede for us, he must have some knowledge of what's going on down here. Are all of them watching us all the time? Is their business about earthly matters? Or, do they go about their business in Heaven until a prayer request lands in their in-boxes? That might work for a while, but what happens when every single person the deceased ever knew has died? There will always be a point when no one is asking for your intercession anymore because no one even knows your name. Too much time has passed. Are they lonely? :) And, with over 5,500 canonized Saints, I'll bet that even some of THEIR in-boxes are pretty bare.
As St. Paul says his farewell to Timothy, his thoughts are not with Timothy's salvation, but with the earthly affairs of the Church, for which Paul foresees hard times. This kind of work is indeed done, and the context does not speak of any mystical intercession.
Are all of them watching us all the time?
Apparently so, see the "cloud of witnesses" in Hebrews 12:1-3.