Coal and the like is finite and will run out. We'll have to switch to renewable energy eventually - the question is how much we'll damage our planet before we do.
The point we should make is even if we weren't damaging our planet the scarcity of coal is reason enough to start early at looking into alternatives so we don't end up with no viable options.
We can never test how much the humans have done to destroy the planet Earth that we are currently living in now but what ever we did, we damaged it and it's in our power to fix it or somehow make an alternative. That's why we need more alternative resources of energy.