With Chalcopyrite (and means for making hard metal) on cave level 2, hearthlings must now build a mine hole before they can access a huge part of the game.
This is going to result in a shitload of requests here that the soft metal/casting system gets fleshed out to include tools people would otherwise need to wait 1-2 months to get. That is one way of making things better, but possibly a waste of developer time - big groups will leapfrog right by any soft metal phase because to them, 10 hardened leather is annoying, but nothing they can't do in a week.
If the aim of the metal changes is to make metal more exclusive and limited to large groups and extremely dedicated players, forcing trade and cooperation, this is perfect. If the intent is to slow down player progression, I think pyres have accomplished that and it would be worth considering scaling mineholes so that from cave layer 1 -> 2 they cost 1 or 2 hard leather, increasing for deeper levels. You aren't likely to see a lot of minehole clutter, because it's easy as shit to bash pyres and nobody wants to waste even 2 hardened leather.