If the US role in Poland is shrinking, then whose is expanding? Germany's? The EU's? Russia's? Israel's? China's? Somebody else's?
The EU's role of course.
In another 20 years Poland will be even more connected and immersed with the other memberstates.
Also in 20 years Poland will (hopefully) having grown out of the developing phase and fully taken over responsibilities as one of the more active core countries, that also furthers commitment.
Should there really happen a groundbreaking event like a split between the north and the south, Poland will be part of the northern EU. Even more tight.
The US will have to fight with their own problems in the mean time, not being able nor willing to spend so much energy on other continents. They will be glad if the EU is still an ally (as we should be).