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Ok. I feel like you did a great job of explaining in detail why you think that way, and I honestly don’t think you’re totally wrong about much of your supporting evidence. I agree with you that something like rapid decline is a very real possibility in our future. But there are two counterpoints I’d make. 
The first is that I think I see local politics a little differently than you do. I’ve spent a good chunk of time working on local issues, and the main assessment I have is that local activity is basically being abandoned. Local government in the United States provides us with really powerful tools to make change in the world, but basically no one is using them. When you start showing up to stuff, you quickly find that there’s a leadership vacuum and people will put you in charge of things by your second or third meeting. They’re desperate for people to do stuff, and this is the case all over the US—big cities, small towns, and so on. 
While that’s a bummer, and maybe a sad commentary on the state of polity, there’s honestly nothing stopping people from simply showing and picking up the tools available to them and starting to use them again. All you need is a will to do it, and a little bit of free time, and you can make enormous change in your community. 
Which brings me to my second point. Everyone alive today (in the developed world) happens to have been born into pretty sweet circumstances where human flourishing has done nothing but improve for multiple generations. Lucky us! But this improvement hasn’t just happened by accident or due to enormous structural forces no one can control. The generations of improvement occurred because generations of people lived through moments where things could have gone either way, and chose to do what it took to move things to a better place. Whether you’re talking about science, or politics, poverty or war, each improvement that occurred happened because somebody lived in a challenging and difficult time and decided to push things forward. 
History is full of moments like the one we’re living through now. I can imagine someone looking at the world in 1939 and thinking exactly as you do now. Fascists on the march, Franco just winning in Spain, world war coming. Who could have guessed that a mere decade later we’d be sitting on the precipice of a 70+ year period of relative peace and prosperity?
It seems to me that nobody has really engaged with what you've said here yet, because you've said a lot. To summarize as simply as possible, this is what I think you're saying: 
1) Human flourishing has improved a lot in the past, and
2) We're at the beginning of a downward trajectory that only seems to be accelerating. 
Does that sum it up well to start, at least in a like very TL;DR way? >

Yes, that would be an accurate TLDR.