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There are two parts to your question.

Humans can certainly continue to thrive on earth whether sea level rises or not.  We live in quite a variety of climates now.  Some climates shifting more towards the poles wouldn't change that.  In fact, polar areas are sparsely populated.  The polar areas that don't get flooded would likely be more hospitable to humans.  There will be less overall land to live on, but there is little reason humans can't continue to thrive on what will be left.

Whether deliberately trying to re-freeze the poles is "justified" is a political argument, and off topic here.  There is no science to speculate on.  Those that live near the coast that lost land, and those that live in polar areas now made more hospitable will probably have different views on how "justified" changing the global climate is.