Leaving changes nothing, it just accelerates the bad behaviour because the obstacle to the bad behaviour removed itself. A single person, even a team of engineers, leaving a FAANG company will have near-zero impact on the behaviour, functioning or profitability of that company. The action will not change management or ownership's perspectives on anything. They'll simply hire someone else, and promote internally if they need too. Even when you have a celebrity engineer like Tim Brey leave Amazon, publicly explaining why, outright slamming some of their behaviour, nothing will change. Mozilla seems to think it can function exactly the same as it did before (in terms of mindset) having let go of 25% of its staff. In the UK we recently had several politicians leave their senior positions within government or their party over disagreements in policy... and nothing meaningful has changed.
Unions are not just for protecting jobs, they're much more about staff having a voice, if not a seat, at the executive table. Unions can help influence senior decision-making. Most of that is about job protection, pay, quality of life issues because in blue-collar jobs those are the key issues. But in white-collar jobs a union can represent the opinion and aims of staff in a way that isn't obvious without them.