Why is Venus hell and Earth an Eden?

Tectonic and volcanic activity release CO2 into the atmosphere. They comprise a major part of Earth's carbon cycle. Over time, carbon would be depleted from the outer lithosphere without this replenishment, and one of the mechanisms by which the Earth ultimately becomes unsuitable for life is a slowing of the tectonic cycle and depletion of said carbon.

This point is addressed (briefly) in TFA:

If or when Earth’s large-scale subduction shuts off in about 3.5 billion years, kneecapping the planet’s ability to bury carbon...

See also:

"Evolution of Earth’s tectonic carbon conveyor belt" (2022)

Concealed deep beneath the oceans is a carbon conveyor belt, propelled by plate tectonics. Our understanding of its modern functioning is underpinned by direct observations, but its variability through time has been poorly quantified. Here we reconstruct oceanic plate carbon reservoirs and track the fate of subducted carbon using thermodynamic modelling....

https://www.nature.com/articles/s41586-022-04420-x>

"How plate tectonics has maintained Earth's 'Goldilocks' climate" (26 May 2022)

(Popular article based on the same research.)

"Timeline of the Far Future"

See "The Sun's increasing luminosity begins to disrupt the carbonate–silicate cycle..." ~500 to 600 my.

https://en.wikipedia.org/wiki/Timeline_of_the_far_future>

This is somewhat speculative, and the precise nature / timing of carbon collapse may differ. But what strikes me is that the various pathways by which Earth exits its Goldilocks state are numerous and comparatively soon on a geological timescale. We're far nearer the evening of Earth's day than its morning, by multiple such measures.

Economically brave. Imagine building the most advanced and expensive piece of technology your society ever created, with small margins of success and the knowledge it'll last less than an hour before imploding upon itself.

And then trying again 16 times.

(I've just learned there are plans to try one more time: https://en.wikipedia.org/wiki/Venera-17)

>By launching your rocket from the ballon city and returning to orbit. (I'm struggling to see the novel problem.)

I believe they are having trouble envisioning how you would launch rockets from a balloon city without disturbing the equilibrium of the balloon city because it is assumed that rockets thrusting down with great force would damage the balloon city in a way it would not easily recuperate from.

I also find the idea difficult to understand, but assume that is because it is in an area I know nothing about and the problems that I think sound bad are actually totally solvable engineering problems otherwise it would not be have been suggested as a solution by engineers expert in that area.

on edit: changed rocks to rockets

With radical ideas it's likely that anywhere can become habitable - there have even been ideas to create floating cities in the thick atmosphere of Venus. But places that require esoteric and ever more complex solutions are obviously less desirable than those that don't. And Mars is 100% 'easy mode' in terms of our first colonization outside of Earth.

Floating on a liquid surface is markedly different from floating within a fluid (liquid or gaseous).

To float on a liquid, one merely needs to maintain a lower average density within the vessel than the surrounding liquid. Assuming a largely hollow vessel (as with a ship or barge), it's possible to add or remove considerable payload without losing stable flotation characteristics as the draft of the vessel automatically compensates for the variation, displacing more or less liquid, and maintaining equilibrium.

To float in a fluid, one must maintain precise neutral buoyancy, which is an entirely different animal. As pressure varies with depth or altitude, the tendency is for a vessel to contract as it sinks and expand as it rises, leading to a runaway buoyancy shift (increasingly negative with depth, increasingly positive with height). Many military submarines operate at comparatively shallow depths, often only slightly more than their overall length* (for larger submarines), given both the immense pressures of even modest ocean depths (a few hundred metres), and the compounding nature and risks of runaway buoyancy loss.

Plans for cargo airships face corresponding problems in that when offloading cargo or passengers it is necessary to vent or otherwise scavenge lifting gas (the expense and/or challenges of either venting or compressing helium are great), or to onboard a corresponding mass of ballast. Where suitable water is plentiful the latter is fairly viable, but there are many applications proposed for cargo airships which suggest transport of heavy cargos sites with limited capabilities for same (no facilities, deserts, salt- or otherwise-contaminated water which might play poorly with buoyancy-compensation systems aboard the airship).

Rocket launch from an inhabited floating atmospheric platform would require accumulation of large stores of fuel (the Tsiolkovsky rocket equation also works against you), as well as presenting various risks associated with enormous barely-contained explosions (should you be lucky). The risks are immense, and hand-waving them away is disingenuous to the extreme.