All that freed, formerly captured carbon comes from trees and plankton. How could burning it all lead to life-destroying conditions when life must have been able to exist under those conditions?
The rapid change in environments can make many species no longer suitable for their environment leading to extinction. Sometimes they can transfer to a new environment, or sustain at a degraded efficiency, but often they cannot if the environment change is drastic enough. This is happening everywhere all at once.
As to the larger point, the sun was ~30% dimmer 4.5 billion years ago and will get about 67% brighter over the next 4.7 billion years. While we would be dead long before we could free up all the worlds trapped carbon, in such a hypothetical situation the world isn’t simply returning to an earlier state.
It's the same principle as what makes a battery fire so destructive - all of the energy stored in the battery in a plant somewhere else suddenly gets discharged all at once in your car.