Well, as far as I know, Lennon wasn't at the vanguard of social change and revolution, neither as a Beatle nor in his solo career, but when he jumped on the bandwagon, he was an effective voice for it anyway. The Vietnam War, and the growing movement against it, was already going on when The Beatles were still making love songs, and yet by the 70s, Lennon was considered a dangerous man and he was followed around by the FBI. Most wanna-be iconoclasts focus only on the first bit and conveniently forget about the rest.
Sure, the guy was a utopian, and sure, he was in a privileged position by any measure, but it would be silly to reduce his work to just that. No matter what else can be said, he did make the case against war and for a more egalitarian society. And in his time frame, pacifism in the west was undoubtedly, objectively a progressive movement, providing aid and comfort to the struggle of the Vietnamese people while destabilising and polarising society at home. Right there, in the belly of the beast, the ruling classes were badly shaken by the events. Even if they managed to co-opt and then defeat this movement in the end, it took them a couple of decades, and there hasn't been such an existential threat to their dominant culture since then.
One only needs to look at the
timeline of events, seeing what sorts of actions were taken, how many people were involved, and how they affected public opinion and, presumably, policy. Today, the Stop the War Coalition gives a platform to Gilbert Achcar, a supporter of the Fourth International who says that the West
is not doing enough to support Jihadism in Syria, while barring a Syrian nun who supports the Assad government. Quite a difference compared to the IMG and Tariq Ali in the late 60s, which Lennon supported.
Lennon certainly was a latecomer to this movement, and he was not the greatest or the most consistent radical in the world, but he still played a prominent role in the end. Just consider his open support for Irish republicans, and compare that to the spinelessness that leftists display towards their government today ("Stop the war, please?"). How is that for revolutionary defeatism? Even in 1980, just three days before his death, he wrote a letter in support of striking Japanese workers in LA, and would have attended their protest if he hadn't been shot.