He's not interested in changing anyone's mind. That's why he blocks people on Twitter rather than rebut their claims (or rather, blocks them and THEN rebuts their claims once they can't respond). He wants reassurance that he's right and they're wrong, even if he's the only one providing it. On Twitter, I suppose he might get a like or two from people who stumble across his stuff without context. I'm guessing Instagram works the same way. And while those little trickles of validation are incredibly gratifying to him, they're optional in that as long as he's repeating it often enough and loudly enough, it must be true.
He's quite sick. But this is how his whole life has probably been, retreating into solitude and fantasy when the world disagrees with him. I think he was genuinely shocked when his Taylor Swift book didn't become a best seller because that was pure distilled Russ-narrative and he can't understand that he actually doesn't dictate reality, no matter how much he explains. It was supposed to exonerate him, make him sympathetic (and famous) and also, once and for all, set the record straight. The rest of the world just needs to understand what (he says) really happened, and then it will become the truth.
It's neat that he's scaling back to making bitter comments on Instagram rather than writing more books, though. It's a purer, more distilled form of reality-bending on his part. You might think a narcissist losing his grandiosity would be progress, but in this case at least I'm quite confident it's a sign of deterioration. He's losing his outlets for spinning his lies and the gosh darn world is just not cooperating. Normal people would self-reflect and maybe ask themselves some hard questions about their lives, but Russ's whole psychology is predicated on him never being wrong about anything, lest of all himself (the awesomest guy in the universe, 9/10 and college educated, goes to the gym and owns a suit). So...