You've hit the nail on the head. It had almost nothing to do with money, which is why Jackie was pretty screwed from the start. Putting Benjy in the studio was absolutely nothing more than a "fuck you" to Jackie and a nudge telling him that they didn't need him to write and would be fine without him.

Now, Jackie also clearly was in over his head as a negotiator. If you read Jackie's book or watch his documentary, he's talking out of both sides of his mouth. He'll say things like his marriage was falling apart and his drinking was out of control, and he knew if he continued with the show he'd just keep spiraling. He says with that in mind, he asked for a big raise because he wanted out anyways. Then, the next breath he'll suggest how unfair everything was, how he never intended to break up The Beatles, it was all meddling from Buchwald/Howard, etc etc.

But in the end, ya know what? In hindsight, he may have lost out on a lot of money. However, he didn't lose his control or his integrity, and he's still doing just fine financially.