The EU population is 500+ million. Even without the UK it will be ~445 million. Russia's is 143 million.
What this means is simple. The EU has the wherewithal to insure its own defense. Hints of a reduction in US commitment to NATO should be taken as an incentive not a threat.
Russia is not the ten-foot tall ogre many are hyping. Even with its natural resources it only has a GDP of $3.5T versus EUs GDP od $18+T, and a population less than half of that of the US. It has a substantial military, yes, and nuclear weapons, but let's be clear. It is not a conventional military threat to Europe, and will be at best a regional power - mostly to its southeast. The fact that Putin has picked off low-hanging fruit in the Crimea and Georgia should be no surprise to anyone.
Since the fall of the Soviet Union, US involvement in NATO has not been about countering Russian hegemony. It's about countering German hegemony.
The real threat to Europe is not Russia. It's not even US disengagement. The real threat is a unified Germany.
Germany’s industrial policy and clout has impoverished the European “South” through enforced austerity and the imposition of the Euro, which makes German exports cheaper than they should be and the exports of Southern European more expensive than they should be.
Germany essentially runs the EU’s monetary policy at this point, a policy which has been in the self-perceived interests of Germany, and only coincidentally in any other country’s interest. (Something the French should get around to noticing, and stop kneepadding for the next German annexation of France, even if not in name.)
Germany is the actual threat to other European countries sovereignty. This might be acceptable if a German hegemon had a record of caring about what happens to non German countries, but the record is clear and visible on the ground in Italy, Portugal, Spain, Greece and even France, that it is not.
The Real Threat To Europe Is Neither America Nor Russia
The relationship between Germany and the other nations of the EU is not one of equality. It's not even first among equals. It's much closer to the relationship between the US and the rest of the Americas - with France playing the part of Canada.