[jorblumesea]

In intelligence, the metadata around the messages are sometimes more important than the contents itself. Parallel reconstruction is absolutely a tool and often law enforcement can get what they need through other means. It's just another puzzle piece in an investigation.

For example:

who talks to whom

when a user comes online, is most active, and general traffic analysis patterns

what groups a user might be present in and how active they are

what are the type of contents of a specific message (image, text, video)

As other users have noted as well, it's unclear as to the data sharing agreements these large companies have with various government agencies. For example if an agency has data access to messages DB, what does end to end matter? End to end encryption usually means un-snoopable data in transit, not data at rest.