Well yeah but I think it's more that their purpose is currently ongoing rather than they'll be needed at a later date. And had Dean been sent to the outreach of space it'd probably take awhile for angels or reapers to find em.The thing is, that's nonsense, too. Sam and Dean are human. When they die, they'll enter the Veil, and then a reaper will take them to Heaven or Hell, as would happen with any other human. So they would actually still be around and be accessible if they are needed, as we already know that angels, demons, reapers, etc. can revive people whenever.