Energy could be harnessed from eternal sources, like the sun, the wind, or the seas. But there is only a limited amount of elements in planet Earth and—what's worse—bringing them from other planets will prove impractical with our current technology (and the technology that will be available in the next century).
In the meantime, copper—which is everywhere around you—will be gone in about 61 years; antimony—widely used in medicines—will be depleted in 20 years; while indium, rhodium, platinum, or silver—which are present in many essential consumer electronics—won't last much longer. And those estimations are only valid if we manage to consume half of what we are consuming now.
So, unless we really push technology forward, dramatically increase our recycling rhythm, or something extraordinary happens first—like Apophis obliterating us or the Large Hadron Collider blows us to another dimension, or Nazi zombies getting out of their crypts to make bacon of all of us—we and our children are going to have a really hard time pushing the world forward.