Sooner or later we're going to have do it anyway, as the sun will eventually expand, vaporizing our pale blue dot. By then, we will be long gone ... literally.

Colonizing Mars would be centuries in the making. I wouldn't see it as an alternative but as a first step toward colonizing our solar system and then eventually leaving it. I think the problem the powers that be have is that death is most probable in the early missions. We could spend billions on that first mission and have the astronauts die suddenly. It's not only the money but the emotional capital that we would spend on it. We would have to take those deaths in stride and few are willing to admit that would be necessary. Tangentially, have you heard about this possible Venus probe.[phys.org]