Life develops outside Earth. Extrasolar perhaps, but we'll say Mars.

"It's an extremely unlikely scenario"

Is it? Seems to be a very, very likely scenario.

the Earth has been the target of numerous impacts by asteroids and comets that have had the capability to eject large amounts of unshocked, and therefore unsterilized, material into interplanetary and interstellar space. Collaborators of University of Arizona Professor Jay Melosh, such as Swedish biologist Curt Mileikowski, have published calculations showing that significant fractions of this material can find their way to nearby planets such as Mars within time scales that are very short compared to the demonstrated viable lifespans of dormant bacteria. ** ...500 kg/ year of unsterilized Martian rocks are estimated to fall on Earth every year.** Zubrin, Robert (2011-06-28). Case for Mars (Kindle Locations). Free Press. Kindle Edition.

What you'll find when you do some research is that bacteria can be incredibly hardy. It can lay dormant for a long, long time and withstand temps, pressures, and radiation that can't be found on Earth. Unless you show me some calculations that indicate that unsterilized matter cannot be ejected from a planet I'm going to go with Mr. Mileikowsi's suggestions.

It's strange that your argument against panspermia is "it's just really unlikely", and yet the alternative (abiogenesis on Earth) is also considered extremely unlikely.

When they do actual tests on things like bacteria surviving in space it's not very likely and all of this ignores the entry through the atmosphere and crash landing.

I mean that's just not true. I'll ask you, is it impossible for the bacteria to randomly detach in the atmosphere before collision with Earth? If you did some research to see whether bacteria currently reside miles up in the atmosphere, what would you find?