I feel like we're working at the same company. Not just this comment but your others on the same topic. I've seen all the exact same mistakes over the last year. The company wants to grow fast so hires quickly, but then the people hired quickly underperform, so then they're fired quickly, but firing people quickly results in fear, grief and guilt for everyone who hasn't been fired "this time". The top talent never feel comfortable in this cold mercenary culture, so they don't settle in and soon move onto somewhere less cut-throat.
That sounds like a vicious cycle: when people are stressed out, they are less likely to be able to learn successfully, setting them up to under-perform, get fired and then further stress out everyone else around them.
Cortisol has never improved a line of code.
Doing an explicit probationary period could at least reassure people who have been there longer, but it seems like it would be hard to regain trust at that point. The company should probably be praying its employees are unionizing behind the scenes & can save them from the mess they are making.