Surprisingly, the existence of humans, or their biological or robotic descendants, is projected to extend Earth's lifespan before it is melted by the aging Sun. This disregards any human efforts to extend the habitability of our planet by building shades or altering Earth’s orbit.
Our system is already 4.5 billion years old, and it’s roughly halfway through its life before it becomes a tiny white dwarf. Before it happens, it will expand and become a red giant. It will then absorb Mercury and Venus. The fate of Earth is less certain because, as the Sun expands, it will also lose mass, causing the orbits of the remaining planets to expand. Most likely, Earth will be devoured in a manner similar to that of Venus. There is a small chance that it will become a burnt ember of what it was if it's spared this sorry fate.
In the run-up to this, many natural systems that make our planet habitable will fail one by one. For decades, we thought we might have only 600 to 800 million years before Earth becomes uninhabitable, and many answers on Quora still repeat this, but recently, we have made some progress in knowing with greater certainty the real fate of our planet. There is even a chance that we might have up to 1.5 billion years before it becomes uninhabitable, assuming that humans or our biological or robotic descendants are gone.
It is projected that as hundreds of millions of years pass from the present, the atmosphere will become increasingly devoid of carbon dioxide, which is used by plants. This will happen because our planet will receive more heat from the Sun, accelerating rock weathering and increasing the rate of carbon dioxide absorption. Also, the plate tectonics, which recycles nutrients over geological timescales, might end.
We currently produce copious amounts of this gas as a byproduct of industrialization. If our descendants still exist, it’s safe to assume that we will still be supplying the atmosphere with this gas. Hopefully, it will not be ruined by the speed at which we are releasing these gases now.
If the lowering of carbon dioxide concentration can be avoided, the next dangerous step will be the increase in the energy Earth will receive from the aging Sun. If our descendants don’t step up and either build a shade or move Earth’s orbit away, then the upper limit of Earth’s habitability will be around 2 billion years.
It will then become so hot that the oceans will boil away, and we will not be able to live on Earth as it melts under the Sun's expansion. This will happen on timescales of another two billion years or so. Although Earth’s orbit would tend to expand as the Sun loses mass, tidal effects and the Sun’s enormous growth may still lead to engulfment.
At some point, its rocks will melt, and a global magma ocean will cover the surface, erasing the entire history of our planet, the good and the bad. When Sun’s surface approaches really close, Earth will at some point start to be dragged by the Sun’s atmosphere, and it will spiral into its plasma and literally melt. This will only be possible to witness from a distance and not from the surface.