I tend to think this, too. Or rather a small group of cryptographers working for a nation-state. It's the only way to make sense of the fact that Satoshi is enormously wealthy. I don't think any individual could sit on this kind of wealth and not cash out visibly.
Satoshi may also be unable to cash out simply because they are dead.
Keep in mind that he could not have cashed out his tokens in the early days without destroying the whole project and by the time btc was valuable and liquid enough for him to sell, he would have already been wealthy from blockstream (if this is really him) and wouldn’t need the money. What would he do with it? buy gold, real estate, tbills? What asset would he ever put the money into that he would think is better than bitcoin?
I tend to agree, but for the sake of argument: it’s possible that he’s such a true believer that he’d see cashing out as a betrayal. Alternatively, he might understand that cashing out would significantly aid the (clearly large number of) people engaged in unmasking him. Further, he may simply realize that liquidating holdings of his size would drastically alter the market in ways that could end with the whole thing coming apart.