If I could snap my fingers and everyone would switch to Matrix [1] I would, but as it stands hundreds of millions of people use these services daily and getting them more privacy will be useful.
They probably can already do everything you say, eg: cross reference and internally "trade" (meta)data between facebook, instagram and whatsapp backends and deliver that to law enforcement. I'm pretty sure their ability to do analytics is advanced enough (though even technical people sometimes really overestimate how "good" those big companies are at it).
At least at that point they will not have the contents of your messages. Which now they do (with the exception of whatsapp).
They will build the largest surveillance system ever conceived and will sell it under the banner of consumer encryption."