Re: Let's start with the basics and then work forward from there.
The Earth is finite.
Therefore, it has finite resources.
Any closed system, the laws of thermodynamics tell us, is a losing proposition; entropy inevitably increases. Hence, recycling what we have here while avoiding "spending billions out there" (...what are we spending our money on "out there"...? I thought it was all paying for stuff down HERE that would GET us out there!) is, inevitably, a losing proposition. It will allow us to deplete our resources more slowly, but just as inevitably.
Meanwhile, "solving" one problem (or, at least, reducing that problem's impact below the threshold where it's seen as a problem) simply raises ANOTHER problem into the position that that one formerly held. Eliminating smallpox simply made some OTHER disease the Number N killer in the world. Were we able to eliminate every disease but the common cold then, by definition, the common cold would be the major killer in public health and so would absolutely HAVE to be solved before we did anything else. This trend holds for ANY group of problems: Demanding that [set of problems] MUST be solved before [other thing] can be done doesn't mean that [other thing] will ever get done if [set of problems] actually DOES get solved; all it means, inevitably, is that [new set of problems] will now be the bugbears that MUST BE SOLVED before we can even THINK about doing [other thing].
...And -- based on the above mentioned laws of thermodynamics -- [new set of problems] will need to be solved with fewer resources than were available to solve the ORIGINAL [set of problems].
Thus, the end result of a "fix everything down here before even THINKING about going out there" mentality is -- inevitably -- that eventually we will no longer have the resources available to us to make the sustained effort necessary and it will be IMPOSSIBLE to leave the Earth at all, since we will have "conserved" and "recycled" ourselves into poverty for all.
OTOH, there appear to be vastly greater resources, in terms of energy, minerals and hydrocarbons, in the asteroid belt, the gas giants, and their moons -- not to mention the sheer amount of new knowledge learned out there and while trying to GET out there -- -- which, if we could but access them, might help us solve those Earthbound problems.