When petroleum does run out the world’s population will have exploded to the point that providing sufficient energy to everyone to sustain a reasonable lifestyle will be impossible.
Even today “alternative” energy sources simply don’t even come close to providing the output necessary to maintain today’s population at the same levels as burning hydrocarbons does.
Quite simply, there is only one energy source that can provide similar energy output levels but its consequences, as demonstrated a couple of years ago in Japan, make it unattractive to most.