I have seen higher tech bows, which most certainly were not made from sticks and string. So yea, this point is invalid.
As for The coal cost, this is one of the infuriating problems with the *no, you may not grow diamonds* thing, with no obvious solution.
No, you may not take charcoal, and grind it into dust, even though in all other ways it is identicle to coal, because it would be far to easy to make a huge tree farm, make a ton of charcoal, grind it up, then start mass producing diamond cheaply (you have to use the far more expencive method of mass fabrication)
So, as a result, anything and everything else that requires coal dust, cannot be made with charcoal either, including fuel, (which would otherwise be a competitor to geothermal, since it's output is probably sufficiently high to make up for the loss in tin) Gunpowder (although you also need to use redstone) Single use batteries (yep, I still make mass charcoal and burn it from redpower trees, because I cannot use it any more efficiently) and Construction foam (also uses non-renewable clay, redstone, and water, possibly tin unless you use a buildcraft setup to make it go at a reasonable speed)
And now this, which would probably be fine if you could grind up charcoal into Charcoal dust, and then use it for everything EXCEPT for making industrial diamonds.