The lower end estimate depends on the specifics of the increase in brightness accelerating the weathering of silicates, leading to more CO2 absorbed out of the atmosphere until C3 photosynthesis isn't possible. Some plants use a different method which will continue to work (C4), but consequences of plant life as we know it dying off would be catastrophic for life on this planet - barring of course, whatever adaptations are made.

But it's certainly the mark of "the beginning of the end" for life on this planet - it's a major milestone that we (the species) do need to leave eventually if we want to continue.