> From many conversations, and my personal experience, the reliability of prime for two day deliveries has gone way down

Also they push Amazon Logistics on you, which is terrible. I've had so many problems with them. The worst thing is they removed the option to "de-prioritize" them for your deliveries, like you can with any other problem delivery service. So you're SOL if you, for instance, USPS knows how to do its job in your area.