Why not? They could use an algorithm that checks if the message looks important, and if it doesn't, asks the sender if it's urgent. They already do it for messages that are out of hours anyway.
Some users prefer simple predictability (with the ability to customise) over 'smart' functions with inscrutable behaviour.
Just look at how often people complain about Facebook/Twitter/Youtube making it hard to see a chronological list of all posts from everyone you've followed.
