> Shouldn't plan B for most of those scenarios start with the creation of self-sustaining, independent ecosystems here on Earth?

Absolutely not.

>I think it's odd that most of the people who argue the "don't put all your eggs in one basket" point, end up with "make rockets!", not "make self-contained ecosystems!"

Putting aside classes of events which scour life from the planet (gamma ray burst, being hit by a really huge asteroid, etc. - admittedly having an incrediblby small chance of occuring, but utterly devastating if they do) where the only mitigation is to disperse your eggs amongst a number of stellar baskets, you still have a number of problems.

To maintain your isolated ecosystem (and they have to be totally isolated for the argument to work, otherwise it's just a remote community e.g. remote Alaska) you have to be able to maintain your civilisational tech level. If you have to be able to sustain yourself, that means every industrial process your civilisation can achieve must be reproducible inside your isolated community. Not only do you have to be able to acquire raw materials in bulk (or make it yourself at incredible cost of energy [1]), you have to have the industrial machinery to produce it along with all intermediate products. And on top of all that, you have to have people who understand the processes involved at every level. No single person can understand that much or even groups less than thousands or tens of thousands eople, from the low level physics, to metallurgy, wire drawing, PCB construction, architecture, various fields of engineering, electronics specialties, agriculture and farming, etc.

No one person alive could make a modern computer from scratch, or even something relatively simple like a (modern) solar cell. You might be able to produce tech up to around WW1 era with some difficulty, or even WW2 if you had a largeish population, but something like building a computer processor using a 14nm fabrication process would be impossible. Even building simple engines for cars would still be useless unless you have the ability to produce engine oil, fuel, lubricants, and so on. People do build wire wrap computers and other curiosities by hand, but that's still relying on civilisation to provide materials and knowledge. And this is just trying to fight off entropy, making no consideration for doing useful research and creating new technologies. Unless you handwave in scifi tech like nanobots that can mine, produce, and repair things for you, the only way to be truly self-sufficient is to occupy a massive footprint of land (continent size or larger depending on raw resource availability), have a massive industrial capacity, and a massive population to not only run all of this but also to educate and perpetuate - so you don't set it all up and then end up losing all your tech in a few generations because no-one understands it.

This is getting rather long winded so I'll stop, but my point is that if you're going to go to all the effort of setting up a Redundant Array of Independent Civilisations, you might as well do it properly and send them to another world where they won't be at risk. Nothing else could justify the expenditure. Not to mention not having to worry about borders, mining rights, mineral depletion, raiders who want to take your stuff for themselves, sabotage, etc.

[1] https://en.wikipedia.org/wiki/Nuclear_transmutation