What is going to happen to Voyagers I & II after the 30 kg of hydrazine is gone?

Voyager 1 has enough hydrazine to keep going until 2040, while Voyager 2's juice can keep it hurtling along until 2034. (Though the spacecraft are identical, Voyager 2 has had to expend more hydrazine visiting Ur and Neptune.) The real limiting factor is the other fuel, plutonium-238 dioxide. Come 2020 or so, there won't be enough plutonium left to keep the heaters working, and everything of value will shut down. They have enough initial velocity to continue out into space indefinitely.