This is a significant misunderstanding of what shareholder oversight does. All the biggest shareholders were humongous asset managers; hard to call that "democratic" governance.
Shareholder oversight is limited to addressing the agency problem (i.e., by firing executives if they egregiously harm profits). It's simply not a "democratic structure of governance", and has nothing whatsoever to do with content or moderation decisions. Even "activist investors" focus on things like dividends and buybacks, not content moderation.
Twitter's stock tanked 12% after Trump was permabanned; shareholders clearly had no say in it, or he would never have been banned.
Since shareholders' main impact is to incentivize management to maximize long-term value, and Musk will now own the majority of Twitter stock himself, the incentive has not changed. Even if Twitter was still public, shareholders would have no say in Musk's decision to reinstate (or not) Trump, for example. And importantly, a company being private doesn't mean it has no shareholders; it just means shares are not traded on public exchanges.