Exactly this. Signing over the minerals would just make Trumps scam even easier and more profitable. He would have feigned support for a short time until he found some other reason, or some other resource to extort Ukraine for. Trump couldnt possibly be any more transparently in the tank for Putin and adversarial to all of our allies. Any who capitulate to Trump are just showing him they are easy targets in the future when he decides he wants more, and he will.