based on our current knowledge, not only of the science that we can extrapolate to the future, but of our technology and how we will apply it

note that that list is all negative things we suspect will happen and include no accounting for our own intervention in matters

I wonder how long it'd take for life to leave after Earth leaves the goldilocks zone. How much life would there be after the oceans have evaporated and stripped into space? If life could evolve to not require water at all, maybe it could survive until the sun physically envelops the planet.

20 comets (or asteroids) wouldn’t even come close to doing it. Events like that can cause extinctions of many ecosystems, by many others will be essentially unaffected. Microbes in the sediment below the seafloor are a good example. They wouldn’t even notice the comets.

The slow and inexorable brightening of the Sun (it's already 30% brighter than when it settled onto the Main Sequence) will eventually cause a runaway greenhouse effect. The oceans will all be converted to steam and the surface temperature will be far beyond what life could survive. After that things only get hotter until the Sun enters its end stages and leaves the Main Sequence.