This line of reasoning never made sense to me. The speed of light prevents there from being calamities that can wipe out an entire star-faring civilization at once. Once intelligent life has spread throughout a region of space beyond a few light years, I don't see how extinction is possible.
Because there is zero proof that species will get past the point where they can start moving between the stars. If you're willing to take it as read that every species that is intelligent (or even most of them) will eventually make it to that stage then yes, your other point will hold as well. But that's a huge assumption that may not necessarily turn out to be true.
> Because there is zero proof that species will get past the point where they can start moving between the stars.
The feasibility of interstellar space travel is an unknown unknown. We have so much more to understand about our universe and our world before we can even cast doubt on whether it's possible or not. When we don't even know what 85% of the universe consists of, much less lack a unified field theory, we just don't yet have the knowledge to make bold assertions on this topic.
Yes, that is yet another assumption. In general any conversation about such stuff should start out with stating clearly what your assumptions are. Since the GGGP started out with interstellar travel as assumed I felt no need to fix that (and technically, voyager qualifies as an interstellar spacecraft at this point in time, even if it is an unmanned one). But you are 100% right, interstellar travel by intelligent beings should definitely not be assumed until there is proof of it actually happening and/or being done.
We are within a century of achieving that capability. The SpaceX Starship is sufficient to expand into the galaxy, even if just by the slow pace of island-hopping through the Oort cloud. We just need to achieve an in-space capability for Starship construction and maintenance, which is achievable on that timeline.
You can not possibly make that claim with any degree of certainty. The difference between going to Mars and going to even the nearest star with a manned spacecraft is roughly equivalent to the difference between 'science' and 'science fiction'.
This IS my field, and I can make that claim. The Starship is designed to operate as a reusable, in-situ refutable vehicle in the regime of <4km/s delta-V trajectories. That is sufficient to get anywhere through staging, using other starships as tankers. It is designed for this.
By analogy it's like pointing out that an off-road jeep is sufficient to get anywhere on Earth, even with only a 300 mile range, because jeeps can carry supplies and/or an extra tank of gas as its cargo instead of people, with staging outposts (gas & maintenance stations) setup and supplied every few hundred miles.
You can make the claim that it is your field, which makes it a simple appeal to authority. But that doesn't make your claim true. There are lots of people in lots of fields that make claims that turn out to be bogus and this - to me - seems to be one of those.
If you honestly believe that we have solved all the interesting problems in interstellar travel and it's 'a mere matter of engineering' then that's fine with me, you're going to have to live with me not believing you.
Best of luck, and - FWIW - I hope you succeed but I highly doubt that you will, the whole 'we are less than a century away from X' stuff is exactly why I say it is science fiction and not science, you can't possibly know the future to that degree.