The only difference is that the number of people that can live in extreme conditions is much smaller than the number that can live in normal conditions.

Fighting against climate change is purely about making sure that the earth can maintain 7 billion + humans. Any mars colonization program will not progress fast enough to let 7 billion humans inhabit mars within the next 200 years.

>It's unreasonable to expect the entire human race to stop what they're doing and focus on one issue.

Nobody said "stop what they're doing" except you. But the usual argument is that Elon Musk is accelerating the progress toward mars colonization and accelerating that progress simply isn't needed because of time constraints that no amount of money can solve. It will take decades no matter what a conman tells you.

As I said in other comments, feel free to waste your resources on the project. But if you're trying to convince others that the project is useful or fruitful (and ask for other people's resources or help on the matter), then be prepared for some resistance.