I see two problems here.
Why cut back space programs instead of, say, military spending or bank bailouts? A fraction of either would put humans on Mars and probes on Jovian moons, and a little more cutbacks we'll have us solving climate change as well..
Also, there will always be a promising new propulsion system on the horizon. When you've built a VASIMIR engine, there will be antimatter propulsion, and then some space-bending engine, and then an Infinite Improbability Drive. When do you stop tinkering and simply get your ass to Mars?