We need to go to space because if we don't, all of humanity will go extinct. The earth won't last forever and also tends not to fare well when it goes 1:1 with asteroids or nukes.
And this is the most optimistic scenario, assuming no gamma ray bursts, bolides, or super-volcanoes strike first.
The US life expectancy dropped one of the largest drops ever (1.5 years) and instead of facing the actual real-life disasters happening, "science enthusiasts" are quoting TV shows about science fiction disasters that might happen in 50000000 years... while the scene outside the window literally looks like this: https://abc7news.com/why-is-the-sky-orange-sf-yellow-califor...
Do we really have 50 million years? Very unlikely. Do we even have 150 years? I'd take odds against, we've just come way too close to all-out nuclear war recently and international relations are not trending in a promising direction.
If we send humans out to the stars, our civilizations will diverge. First there's a hop to Mars, then Titan, then then next solar system over, then a galaxy or two. Soon enough you've seeded the universe with human civilizations that have no concept of what Earth was like. Maybe they've heard stories, seen videos or pictures, but what will they really know about Earth in 100k years?
It's not like we're going to be one happy galactic human family. We can't even do that here on Earth, how are we going to maintain peaceful relationships across the universe? So now all you've done is seed the entire universe with a race of petty, greedy, selfish, war-prone primates; living on barren planets, underground or in habitats; who have stories about a mythical homeland filled with a breathable atmosphere, rivers of drinkable water, abundant lifeforms that roam the surface freely, where no one has to live like hamsters (what even are hamsters?) in a habitat. What if they start talking about a better future for themselves on another planet, and they come right back here to conquer it?
I guess what I'm saying is, if all we're doing here is using science fiction to project out millions of years into the future, why are we only projecting positive scenarios where humanity heads to the stars and saves the human race from obliteration? What if instead of inventing the United Federation of Planets, all we do is invent the Reavers from Firefly, and we end up the authors of our own demise?
Mars won't ever be even as habitable as a thoroughly fucked up earth. It is a huge waste of time and resources that could be better employed on more immediate problems.
If we want to keep the earth habitable by humans over long time-scale then we absolutely need to learn how to do minor terraforming. Much better to experiment on the moon or mars than the earth.