I think people misunderstand this goal. It's not 'backup' as in, 'ok, Earth is toast - everybody go live on Mars now.' One of the thing critics get right is that in the overwhelming majority of species ending events on Earth, Mars would still be less hospitable than Earth would. For instance take a massive asteroid impact. It's not the impact that kills you, at least not most people.
But what would happen following a major asteroid impact is a massive amount of matter entering into the atmosphere and effectively blocking out the sun. This results in plantlife dying off which then results in the rapid death of everything on up the foodchain - we starve to death. Yet you'd still mostly be able to breathe the air, your blood wouldn't boil on atmospheric exposure, and so on - it'd still be a rather more pleasant place than Mars.
What Mars can offer is (1) a parallel civilization that can continue on and (2) a lifeboat to Earth. People can return, help reorganize systems of governance and restore order, rescue survivors, and generally get started rebuilding Earth in the case of a mass extinction event. "All" we need from Mars is for it to be relatively self sustaining. I say relatively in that it can provide for the basic necessities - food, habitation, energy, reproduction, and maintenance/repair/replication of those basic necessities. Everything else is a luxury.
And the timelines for that are far closer, even within our own lifetimes. I think this will become more clear over the next decade. China has generally been quite conservative with their space goals and overperforming, and their stated goal for the first crewed mission to Mars is 2033, and every 2 years afterwards to follow indefinitely, as part of a plan to establish a permanent presence on the planet. The first Starship launch to land on Mars will also likely be a game changer for people.
2033 is really not conservative estimate for crewed Mars mission.
It's simply a flyby, not a landing, which will likely happen in 2035 for them. NASA was laying out various plans for exactly this in the 60s, with a timeline of the first crewed flight to Mars somewhere between the mid 70s and early 80s. And it was completely viable. The only reason this didn't happen is because Nixon defacto cancelled human spaceflight in 1972, in part because he was worried that a loss of life in space would imperil his reelection chances. So we get to live in the timeline where space stagnated for decades.
The only fundamental tech we're missing is a heavier launch vessel, which we've already developed in the past - and have actively in development in the present via Starship. China is also developing their own super heavy vessels. But these developments taking 8 years is quite conservative. We went from practically nothing in 1962 (having only just put a man into orbit, and barely at that) when Kennedy gave his to the Moon speech. 7 years later in 1969 - we'd be landing on the Moon. And that landing posed far greater difficulties than just an extended flight, let alone when they were building from nothing, and we have all of this knowledge and prior experience to build from.
A Mars flyby?? With humans? I don't doubt it, but at the same time I can't imagine spending months in a ship just to look at your destination without ever getting out. Talk about cabin fever.
Haha, well depending on the exact ship they go on they'll probably have substantially more room than the ISS. That was the main motivation for things like people staying 370+ days on the ISS. And long before the ISS even existed, the USSR was also actively pursuing this. In 1988 Valeri Polyakov stayed aboard the Mir Space Station for 240 days. His first words after landing were, "We can fly to Mars." [1]
After that he spent a whopping 437 days on Mir (which had about 1/3rd the pressurized volume of the already claustrophobic ISS) to see how the human body would respond to long-term duration in minimal gravity. Upon landing back on Earth this time he decided to get up and walk from the capsule to his rest point (astronauts are normally carried/rehabbed due to muscular atrophy + dysfunctional balance/orientation, even for far shorter stays), making a point of the fact that he was just fine. Dude was just a complete badass. The USSR would have beaten us to Mars if they hadn't collapsed in 1991.
In any case, it's probably a good idea to do a flyby because there will be, with near 100% certainty, some thing things we hadn't considered and others that we simply were not aware of. By first doing a flyby and then a landing you increase the chances of success. And the people doing the flyby will probably be mostly the same people doing a landing a couple of years later - so it'll be more like "See you soon."
[1] - https://en.wikipedia.org/wiki/Valeri_Polyakov#Cosmonaut_care...