I think it was a great idea, useful in many cases (confirmations, approvals, votes, unsubscribes, forms, up-to-date flights and bookings info etc etc. I for sure admire that I can accept meeting invites right inside email in Gmail. Email must stay the same - it has HTML/text parts, these will stay same and can be accessed in 5 years. If someone really wants to include dynamic info in email they will do anyway- either link to webpage or dynamic image.