> all panspermia does is layer on an additional series of long-shot coincidences
Disagree. There is a lot more of not-Earth than Earth. I agree that the probability density (per unit volume of the universe) of life originating on Earth is much higher than anywhere else, but there's just so much more room out there for life to originate that the probabilistic cost of traveling to Earth is tiny in comparison.
All you need is a proto-life that's stable in an inert environment with sufficient radiation shielding. It could've originated billions of light years away and still have had enough time to arrive on Earth 4.2 billion years ago. That's a mind-boggling number of Earth-like environments.
In fact, proto-life doesn't even need to look like Earth life, so even environments that are hostile to current Earth life today could've been the cradle of origin (as a wise man once said, "life, uh, finds a way"). Additionally, environments that used to be Earth-like but eroded away are candidates too since all we need is for life to have escaped before the erosion.
In my opinion, panspermia is strictly more likely than Earth-bound abiogenesis.
Could early single celled organisms have come from an asteroid? Perhaps, but almost certainly not. It is strictly less likely than Earth-bound abiogenesis. Life still had to spring from nothing somewhere - all panspermia does is layer on an additional series of long-shot coincidences.