Remember that Golden Oldie? How do you know that our armed forces are adequate for the future? They have been steadily reduced over time, primarily by Democrat administrations, until we no longer have the credible capability to fight two "peer adversaries" at once as we did before. China, Russia, Iran and others have ramped up their armed forces and their technologies at a very rapid pace and thanks to Obama's reductions and degradations (actively serving gays and girls for the infantry) our forces are further hobbled.
We have been focused on long-term limited war to hold off the immediate threats and have reduced our forces available for forward deployment, which serves to support allies and to reduce reaction time against the larger potential adversaries.
So in answer to your proposal, no, "faster, lighter forces" would not be adequate. And no, Europe or Japan or any of the rest of our allies could not survive long against Russia or China without us.
The Romans disappeared because they cheaped out and manned their perimeter with foreign forces ("barbarians") and when the Attila, the Visigoths and others showed up, they stole everything that wasn't nailed down and had their way, as it were.
We must be a major military power - or we have to accept rule by others.
If I could snap my fingers I would get rid of the Army and give that funding to the Navy, the BP/CCE and build a big beautiful wall.