And nato was created to control and prevent the influence/expansion of USSR. Russia is not that threat anymore, so no major reason for nato to exist at all, let alone expand in Ukraine.
realistically China+Russia is that threat to the West today
It was built to provide collective regional security, in large part by binding the Western powers, who have frequently had disputes and divisions progress to war among themselves, together in an alliance wherein that would not occur. The USSR was a major immediate concern at the time NATO was formed, but regional security concerns did not evaporate when the USSR did.
And probably reinforced the idea of a EU defense system as well. The EU can't rely on US military support indefinitely, particularly since that support may vanish at the whim of the American voters.