> a plausible path to shipping canned apes for a brief sojourn

The important part of that end is a much more economical tech for putting mass into orbit. I'd argue that is currently the single most effective thing that we can do toward becoming multiplanetary. Yes it still leaves a very long road ahead, but it's a bridge across an otherwise impassible chasm. A related Heinlein/Pournelle exchange:

  Unfortunately, in Blowups Happen a capability for orbiting large payloads had been developed. "Aha," I said. "I see your problem. If you can get a ship into orbit, you're halfway to the moon."
  "No," Bob said. "If you can get your ship into orbit, you're halfway to anywhere."
  He was very nearly right.

https://space.stackexchange.com/questions/35015/where-did-he...