I was just at a talk by Nobel Laureate George Smoot talking about cosmology. The new found dark energy which dominates the Universe causes the Universe ot accelerate, effectively beating the slowing down due to gravity. As the Universe expands the microwave background will cool even further and so not only would it be difficult to detect but harder still to get detailed information on it. Current observations of the microwave background are helping pin down so key cosmological facts, how flat the universe is, when galxies formed, how much dark matter and dark energy there is. The timescales we are talking of are in billions of years so I don't think our ability to learn more about cosmology will end very soon.
Cheers - David