_instead_ is the word I have a problem with. the idea that we must do one or the other. that either we fix climate change OR we go to Mars.
But the other thing is energy. Our entire society depends on fossil fuels, which are not only causing the climate problem, but are also limited. Planning to go to Mars in 50 years assumes that we will have enough energy to live on Earth and to launch rockets. Let's be honest: right now we don't have a replacement for fossil fuels.
We mostly have hope, based on "promising tech" oversold by startups surfing on the green economy idea. Wind/solar is growing a lot, but thanks to fossil fuels. Nuclear plants take a lot of years to get built, and in many places in the world we are more closing them than building new ones.
So it is quite likely that we will have a critical energy problem before we manage to send people to Mars. The climate/energy problem is not something that we can solve later, because we are slowly running out of time. And it feels like all the resources that we put in advanced projects like Mars could be (or even should be, at this point) put somewhere else. Because that's a lot of money and competent people. Also it goes with over-consumption: it's not that one such project in itself is too much, but rather that we should start cutting into many of those superfluous projects, to survive.
Meanwhile life on Earth is dying, because of the human activity. Some things are difficult to change (forcing everyone to use bikes all the time would be a big problem: how do you bring food into cities without trucks?), and others seem very easy: just don't put all those resources into useless Mars, and instead try to save life on Earth.