Why is Venus hell and Earth an Eden?

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.

The Sun will get hotter over time and expand, eventually descendants will have to decamp to elsewhere, like Mars.

Almost the entire length of the linked article is spent answering that question in detail.

Lots of reasons for this. The Moon is a complete hell hole - 2 week long nights, night time temperatures that drop to around -200f, daytime temperatures in excess of 250f, extremely low gravity (to the point that one might expect a higher impact of the countless health consequences of 0g exposure on humans), minimal material resources and so on. There is also no atmosphere at all which complicates landing, means even the smallest micrometeorite will impact the surface (which is why the Moon looks like it does), and contributes to highly dangerous particular dust everywhere - much worse than than on Mars.

By contrast Mars is bizarrely similar to Earth. It has almost identical axial tilt resulting in a similar seasonal cycle, a similar annual cycle, extensive mineral resources, some atmosphere simplifying landing - providing protection from meteorites, etc. It has temperature ranges that, like Earth, vary wildly due to seasonality, but are locally consistent. For instance on a summer day near the equator, it hits about 20C on Mars. If not for the blood boilingly low atmosphere, it'd be down right comfy.

Anyhow, kind of a rambling disorganized comparison because I'm in a rush - but yeah, Mars is almost eerily Earth like. In that if life was a video game Mars would be the kind of obvious 'next level', to a degree that makes it feel scripted. Even some chemical reactions like the Sabatier Reaction [1] (Martian atmosphere + electrolyzed ice => methane + oxygen + water) just feel too convenient to be true, but they are.

[1] - https://en.wikipedia.org/wiki/Sabatier_reaction

Yeah, it's full-on fantasy. Why would we as a species waste time terraforming a planet proven to let its atmosphere evaporate into space? Why waste energy to drag materials from Earth there instead of spending the same energy and materials to fix whatever problems Earth has?

At least in a billion years we can expect we would be either extinct already from our own actions, or hope to be advanced enough as a species to move Earth's orbital path out a touch every couple millennia to keep us in the Goldilocks zone.

Maybe by then we can terraform the Mars by crashing a few dozen comets and detritus from the asteroid belt into Mars to keep the Martian iron core, add heat enough to keep it molten and spinning for a while, add enough mass to get the gravity about 9.8 m/s2, reboot a tectonic cycle, combine 2 satellites into 1 good one, and try to add water to the system overall.

You know, just a regular Tuesday for whatever species we evolve into.