ubiq: Currently both Russia and Ukraine claim Crimea is their own sovereign territory. If Ukraine joins NATO today, any escalation of this territorial dispute can result in military conflict, which would automatically invoke Article 5 of NATO’s mutual defense pact, obligating the US and all NATO members to defend Ukraine against Russia.
As far as your second point about the fluidness of European borders, if your attitude is simply “Things change” and “in 1954 Crimea became part of Ukraine. Shoin! fertig Mazel Tov”, then presumably you’re being consistent and also saying “in 2014 Crimea became back part of Russia again. Shoin! fertig Mazel Tov.” And, even, in 2022 Kiev might become part of Russia, Mazal Tov!
Right?