The excuse of China/Russia is beyond nonsense.
1. As part of Nato Greenland is already protected by such aggression. The idea of anybody triggering WW3 over Greenland is ludicrous as anything interesting there it's easily obtainable just by...putting money on the table. I'm sure Greenlanders will welcome royalties while you mine or do whatever in such a terrible environment.
2. Neither of the two has any military capabilities to conduct military operations in such a place and so distant from their countries. So close to North America moreover. Just to put it in perspective: Greenland is closer to Boston, and by a significant margin, than it is to any major Russian port.
3. While Russia has a long history of aggression and territorial expansion, anybody who's intimate with Chinese history knows well that an aggression on Greenland makes no sense and does not fit any pattern seen in the history of China.
The countries that have legitimate reasons to worry about Chinese aggression are those that were part of China in the pre colonial era: Taiwan, Kyrgyzstan, most of southern Siberia and Mongolia. This is why, e.g. China has annexed Tibet, but never gave two damns about neighbouring countries it could grab in an afternoon like Nepal: never been part of China, they don't care.