My favorite fact about Mars is the soil fucks with our hormones. Children born on Mars won't be able to survive because the dust will literally destroy their ability to develop. Shit is haaaard.
Now that's happened all over just the past 4 years. What do you think of our chances 20, 50, 100, and more years in the future hold hold? It's pretty easy to see plenty of timelines where we do eventually manage to kill ourselves, and so expanding beyond Earth is critical. Of course that doesn't mean we should neglect Earth in any way, shape or fashion. But with 8 billion people on this planet, we can manage to do more than one thing at a time to quite a high degree of competence.
Wiki has other details https://en.wikipedia.org/wiki/Martian_soil
In any case, point is: We need to stop being a threat to ourselves, and hoping that another planet, for which we're not evolved to survive on, is a better choice feels like the wrong path. Humans are amazing at surviving... on earth. Because we evolved to survive here.
Having a "plan B" is not a bad choice, because a lot can happen that we cannot prevent. However we can't resettle everyone. Most will die. Plan B is to continue the species, but not necessarily to actually save us.
I also completely agree with everything re: Earth. But one thing I'd add there is that I think humanity actually starting to settle Mars will dramatically shift the mindset of both people and countries. Imagine China was engaging in mass transit to populate and build up New Beijing. It's not like the US would simply stand by. And today I think many people are not so interested in these ideas, because they think it's simply impossible. I'm thinking more like New World parallels rather than Space Race.