Every update always has an impact on your battery life. An iPhone’s battery life is highly dependent on software tricks and background management systems that sleep and terminate processes without you knowing, based on the app and your usage of it. Updates throw a lot of “known” information out of wack and it takes awhile for your phone to sort it out again. This is not even counting the fact that there might just be bugs causing apps to use more battery than they should be when you’re using beta versions of iOS.
I think they absolutely know how willing their employees are to quit. It’s been demonstrated over and over again in the tech industry for the last couple years. It is far more likely that they’re counting on it, than are somehow all being blindsided by it. Suggesting that the latter is the case would be a… wild and practically unbelievable assertion to make.