We are going to need an Earth 2, a real Earth 2, before too long.
We are making Earth 1 uninhabitable, the same as they did.
The question is, how many people does it take to maintain a
technological civilization?
For example, to make integrated circuits you need the machines to make
the machines to make the machines to make the machines to make them.
You need to mine the ore, process the ore, smelt the ore, to make the
steel (or other metal) to make the machines to mine the ore....
etc.
Would 50K people be enough? 1M?
How are you going to get that many people to Earth 2?
Or even Mars?
And, BTW, a child born on Mars or any other planet with a much lower
gravity that Earth 1 will never be able to go to Earth 1. Their bones
will be too thin.
There will be the same problem with people who go to Mars and stay there
too long.
Indeed, after being weightless for the 7 months to get to Mars (best
case) the crew might not be able to stand up even on Mars.
And since they probably won't have a dentist as a member of the crew
don't forget to bring some dental pliers with you. That will be the only
way to solve dental problems.