The problem with geo-engineering is that it only treats the symptoms, not the root cause. Treating the symptoms has a side-effect: it lowers the incentive to treat the cause by making it less pressing.
What we want to avoid is not a 1.5 degrees warming, it's a 3 or 4 degrees one. By doing geo-engineering, you could fix the issue in the short time but you would also increase the probability of a bigger problem in the longer term.
Making it optional doesn't solve the issue since it means those who choose to consume resources eternally at the expense of others are rewarded. In the meantime, those who choose to do the "right thing" (dying a natural death) would be punished: indeed, dying in a world where others live forever seems far more painful than dying where nobody has a choice.
There is no such thing as "doomed". Humanity is not going to disappear. We can either do better or worse, and there really is no limit to the "worse" side.
What we want to avoid is not a 1.5 degrees warming, it's a 3 or 4 degrees one. By doing geo-engineering, you could fix the issue in the short time but you would also increase the probability of a bigger problem in the longer term.