Even if all of Mars was habitable, it would only add one quarter additional Earth surface. But it isn't. And the rest of the solar system is even less hospitable than Mars.
That's why in the long run, we've got to get out of the solar system and plant colonies somewhere else. Mars isn't an interesting end destination, but getting to Mars would help pave the way to start sending colonies out to all those exoplanets we keep discovering, and hoping one of them is habitable.
This may actually be impossible. I've been working up a novel whose premise was a completely believable last ditch survival shot at colonizing the stars with as little new technology as possible. What if some dinosaur killer event is going to happen in the present time, and we've got to just get it done? My novel stalled out, because I'm going to have to invent far more magic than I wanted to make this remotely believable. The problems are immensely daunting, and we're nowhere close to being able to solve them.
We might never solve them. If we don't, our extinction is 100% guaranteed unless some friendly space faring neighbors give us a lift off of this rock. We won't go extinct in the foreseeable future, probably, but it's inevitable that we will go extinct eventually.
Much as we are hard wired to kill our neighbors, fuck their women, and take all their shit, we are hard wired to survive. We want to live forever, which is why we have invented so many religions that allow us to pretend that we get to do just that. To anyone who truly understands the inevitability of the sun going red giant and scorching our home planet, remaining entirely on this one rock runs counter to the innate human survival instinct, and the innate raping and plundering instinct. Thinking about how to get us off this rock isn't a flight of fantasy, it's a compulsion, and the most critically important problem we need to solve in the long run to ensure our version of eternity for ourselves.
Do we accept extinction, or do we figure out how to get this done? Personally, I don't want to accept extinction. A very small minority of us have enough foresight to agree, while the vast majority are playing shuffleboard on the poop deck of the Titanic saying the sun will be around forever and ever, and anyway, Jesus will save us, so why invest in all this crazy space fantasy when we could be spending money on yachts and luxury cars instead?