I don't think this really ameliorates the issue much; it just pushes the problem down the line. IMO this is a big part of why people transfer internally to diff

I don't think this really ameliorates the issue much; it just pushes the problem down the line. IMO this is a big part of why people transfer internally to different teams so much at Amazon, and that masks the problem even further. If by six months of on-call rotation you expect someone to be self-sufficient, but they only start after six months on the team, they'll have been on the team a year by that point, and people either transferring or leaving the company after a year on average isn't going to be immediately obvious as a problem, but if people start a few weeks in, you're going to have 5-6 months of noticing that there are issues when that person is on-call, and if that happens more than a few times, the trend will be noticeable.