We can dare to dream of the stars again, if only we can achieve this small thing.

[Angrboda]

If I recall my specifics properly, there is no intention to replace the shuttle. During the Bush administration, an assessment of NASA's strategic goals, and how to most effectively realize those goals was conducted. If I recall correctly — don't quote me — the fallout included shifting away from reusable craft for most missions, as disposable rockets were more cost-effective; separate payload and crew for most near earth missions, as crew launch is much more expensive and difficult than simple cargo missions; shifting away from an emphasis on manned missions, there being advantages for both near earth and deep space missions, including safety, cost and feasibility (deep space), and general technological complexity. I may have missed a few, as I haven't read on it in a while.

Anyway, space travel is only one way to extend the usable lifespan of our extended biosphere, and eventually all strategies will fail, though perhaps not for a long time. I'm told there are potential technologies for interstellar travel, but most result in one way trips. (And probably the most promising is some form of light speed transmission of the information necessary to construct our mind out of some locally available technology at the other end. For the most part, eventually being a citizen of Earth, for all but maybe a few percent, is going to suck big time. And because of the one-way nature of foreseeable technologies, space travel won't improve their lives except indirectly via basic research in zero-G, advances due to advance of the cosmological sciences and the like. Holding out for moving off-world like the locust-like species in ID4 is probably rather optimistic.)