Not a Rant About Microsoft’s Plan to Stop Old Exchange Servers Sending Email to Exchange Online

Microsoft's plan to stop Exchange Online accepting email from unsupported Exchange Servers caused a lot of fuss and bother. Looking through the commentary and questions about the announcement, I'm not sure if people understand fully what's happening. It seems clear to me, but as Richard Campbell of RunAs Radio fame wants me to rant about the topic, here's my opinion.