I have been part of many online communities. To name a few, the old Usenet, Slashdot, IWETHEY, Perlmonks, and HN.
I long ago accepted that all die at some point. And something new will be created. The new will never be the same as the old. But the fundamental problems are social, not technical.
Your mirror platform is an idea that I've seen tried multiple times, and I've only seen work once. That was when the InfoWorld Electric forums broke, and the core participants moved to IWETHEY as a replacement.
But, generally, the technology isn't what breaks, it is the social dynamics. Usenet, Slashdot and Perlmonks are still out there. They just don't have the same social dynamics that they used to. People enter, others leave, and the community changes. Those who are motivated may agree to meet over there when this goes. But they are always more motivated to be here here now. People who leave here, don't go there because there is nobody there to attract them. And they don't want to form a community over there for a future mass migration because they (probably accurately) project that the people they don't like here will just be part of that migration.
I'll cry a tear if HN falls apart to the point I don't want to be here. But I'll also accept it as just how online communities work. Maybe I'll see people I know in my next community. Maybe not. I'll keep a few personal friends, and I'll move on.
This is just how things work online. And there is no technological solution for what is fundamentally a social problem.