Denmark likely underinvested.
However, there has never been any opposition to the U.S. investing more in Greenland.
There's been a partnership between Greenland/Denmark and the U.S. for 20 years already. Nothing prevented the U.S. from investing more than it did. U.S. mining companies are already present.
https://dk.usembassy.gov/u-s-greenland-joint-committee-statement-october-25-2024/
About defense issues, Greenland and Denmark are part of NATO and there is already a U.S. base. This base's location is de facto locking access to the Northwest passage. I am not aware of any proposal to extend U.S. presence in Greenland having been rejected.
Of course, Trump is lying about China and Russia. China has been banned from any infrastructure project since 2018, and an FDI act has been voted to control foreign investment in infrastructure, energy and minerals. Russia is not interested in Greenland because it already controls the Northeast route.