I almost missed an Intl-Dom connection in Atlanta last week. In fact, if I didn't have Global Entry, I would have missed it. My scheduled connection time was around 90 minutes.
I wouldn't have put the blame on Delta if i had missed my connection, though I certainly would have been upset about it. I am the one who made the decision to book the 90 minute connection.

Was the London flight one of the last flights to Europe of the day? If not, it seems like Delta may have been able to get you on a flight to somewhere in Europe (AMS, CDG, FRA, ...) if there was room, and you could have gotten to London at some point on the 19th.