The blue line with a steady growth rate looks simple, but in a limited environment it is not possible. Finite space does not allow steady influx. For instance, if people were continuously entering a restaurant, it would soon be crammed full. This is true for all limited spaces.
The third line starts slowly and harmlessly but then it accelerates and shoots up. It can only collapse in the end because of the natural law we have mentioned about the borders of space. This is even more true for accelerated growth; the borders of space will be reached faster.