A bad manager can turn a great employee into a good one. It’s really hard to go back once that happens.

I'd go further: a bad manager can turn a great engineer into a very bad one. People look up to great people, and when the strongest performers are demotivated, that spreads.

Commonly in the cultures that end up this way, leadership blames / gaslights the ICs. It's toxic and honestly kind of heartbreaking.