Why would Ukraine in NATO mean perpetual peace in Europe?
I see Ukrainian membership in NATO as a pipe dream; a diplomatic bargaining chip. There does not appear to be any concession on Ukraine or Russia's part which would allow for a mutually agreeable peace agreement between the two.
Because of that, the basis for conflict - proxy or otherwise - will continue to exist in Ukraine. Ukraine's current circumstances make it too geopolitically complex for it to be a NATO member; it puts too much risk on the current members, and the relative peace we have become accustomed to.
Our Ukraine-centric foreign policy in the US and West is only a reaction. We didn't have an interest in Ukraine before, and more than what is foreseeable would have to change to get to such a point.