In real life, we can always make more iron (or insert other resource we might be lacking) by smooshing smaller atoms together. That doesn't mean it's viable source for any scale of use.
If you really need to speed up the tree farms, even though they're a lot better than harvesting wild trees, get bonemeal by creating a trap around an enemy spawner. Or just create a giant dark pit where water feeds the monsters into a trap in the center (either a pitfall or a lava blade). Again, it's all on the wiki. Infinitely sustainable. I've made all of these, except for the enemy trap, which I'm still working on... by using the cobblestone generator.
This can produce cobblestone as fast as you can mine it, at a cost of three lava, three iron (for pistons), and a bit of redstone dust (plus items derived from wood, water, and cobblestone, which are all renewable resources)
There are other designs which use even less non-renewable resources as well.
Still, I have a hard time seeing any Minecraft scenario under which "there's not enough cobblestone" is a valid complaint. Even without using cobblestone generators, it's incredibly abundant.
The other resources I listed (wood, water, charcoal) are all renewable in a practical sense.