Th SDF is more than capable of defending Japan. The US doesn't trust them to be able to do that though. The US gets good economic prospects from Japan, as well as coveted bases of operations that the Western world has salivated over getting since the mid 1800s. Japan gains a double defense from this as well. Also, being a non-NATO signatory, the SDF gains a lot of their technology and intel through the US from NATO.
However, the instrumentality of the US forces being based in Japan can be questioned. I mean, having security forces on tour of duty in Japan is okay, but US bases shoul be turned into Joint Forces bases. As in, both the US and Japan own, operate, and maintain them. I would like to see that setup more than a physical euphemism for occupying forces.
America fosters a culture of smugness, of exceptionalism. They claim to be the protectors of freedom, and of free speech, while actively trying to supress free speech on the truest medium free speech has ever known; the Internet. That's not to say all Americans are in the same boat, but the American brand of culture is very hypocritical, vain, and ignorant. Not to mention very insensitive (I.e: "go kill yourself.")
Deep down a majority of Americans really do think they are superior to all other nations and people. Most just don't let it be known (and of those, most don't know why they are hiding it). It's a sad concept, but most think it is objective FACT that they are better than other nations. It's a Viking/ Gladiator/ Mongol concept of "the strong are better than the weak, we only let people be because we don't want to wipe them off the planet just yet."
That concept has held true throughout the history of the line of Empire. That's why Japan lost its tentative place ad the successor of the Angle dominated empires, and why the brutal PRC is the more likely candidate for the next dominant empire. After that, it's going to be the next nation that develops weapons capable of destroying everyone even worse. Spiralling into self destruction. It's sad that increasingly the most universal human concept is obliteration. Maybe that's why everyone wants a structural collapse or even a full bloody apocalypse.