More open water means ships can move through places that used to be protected by ice. This would have impact on marine life, fisheries, coastal life.
I don't think this will make much impact on the EU allies The memory of the made-up WMDs in Iraq is still fresh.
China aggressively projects power, with a willingness to spend a large amount of effort (sand islands) for footholds. Notably, by the time the evidence of these artificial islands was observed, it was too late to reconsider. Russia does the same, when they have the resources. The US is not used to doing this and has historically fared poorly. Ofc there is worry.