The thing is, the reason so many people aren't joining the military (well there's quite a few, but one) is that they don't see it as actually fighting for freedom. They see our recent wars and go "I don't want to die in some random middle east country I couldn't find on a map."USED TO BE two major wars and a minor "conflict" simultaneously in different parts of the world. I don't think we have that capacity any longer. At least, not from a "boots on the ground" perspective.
Fighting against Russia or China directly or in practice would likely actually lead to a lot more people joining.
Keep in mind, though, our military has likely been spending to compensate for this. And they likely have insane weapons or technology nobody knows about. Or will very soon.
We can do a lot with our drones, likely even crazy drones we dont know about. The military has all sorts of tricks up its sleeves for when it really needs them.