In fairness, the basic premise of NATO was that the US would handle military protection of Western Europe in exchange for solidarity on foreign policy, trade, and economic development.
If Nato didn't exist its debatable whether western Europe could maintain their existing level of defense expenditure. The EU would also need to manage increased defense expenditure amongst member nations to avoid an arms race.
If Nato didn't exist, Western Europe could focus on building defending forces against Russia rather than spending their defense budget on expeditionary forces that need to tag along on American-led adventures like Iraq or Afghanistan.
If Nato didn't exist its debatable whether western Europe could maintain their existing level of defense expenditure. The EU would also need to manage increased defense expenditure amongst member nations to avoid an arms race.