I think you may be missing the proposed use case of private blockchains. They aren't for storing data within a single organization on trusted computers controll

I think you may be missing the proposed use case of private blockchains. They aren't for storing data within a single organization on trusted computers controlled by that organization. They are for sharing data across a limited consortium of organizations who don't trust the integrity of each others' computing platforms. Also, it's not clear to me whether you're asking about PoS security too, but if so, the reason one person can't go back and rewrite a bunch of it is because the right to create blocks is spread out randomly across the entire active set of stakers. So if an attacker only controls a small fraction of the total stake, then their signature would only be valid in building a small fraction of the chain they would need.