TL;DR the sun is expected to continue brightening, and that's likely to cause the end the carbonate-silicate cycle in a few hundred million years, which will mean the end of photosynthesis soon (for geological values of "soon") after that starts to grind to a halt. The end of photosynthesis means the end of large, complex life-forms on earth, permanently, forever, never to return.
This means that the practical end of the Earth, as far as humans are concerned, is on track not for a few billion years from now when the Sun expands—the commonly imagined end of "Earth gets charred/absorbed by the Sun"—but a few hundred million years from now. Maybe sooner for a variety of reasons, but that's likely to be the final lights-out—the upper bound—for everything but some microbes and a few lobsters around deep-sea vents or whatever. Barring some serious intervention by intelligent, technologically-advanced life, anyway.
It isn't the poster who's developed this theory. It's a matter of chemistry. It's possible life could persist, but we don't know of chemical processes that would make it possible, so we shouldn't expect that it can.
This means that the practical end of the Earth, as far as humans are concerned, is on track not for a few billion years from now when the Sun expands—the commonly imagined end of "Earth gets charred/absorbed by the Sun"—but a few hundred million years from now. Maybe sooner for a variety of reasons, but that's likely to be the final lights-out—the upper bound—for everything but some microbes and a few lobsters around deep-sea vents or whatever. Barring some serious intervention by intelligent, technologically-advanced life, anyway.