I don't think adding general purpose and/or secure telecommunications functionality is going to help a microblogging service to be more profitable. There are already more mature tools for that, it's expensive, and people don't want that from twitter. It's particularly ill-advised when you've lost a large number of staff in an uncoordinated way and seriously alienated advertisers. The company needs to focus on making sure the service is in a position to be stable, and then work on repairing relations with advertisers.
I find it interesting people use Twitter for DMs. Will fully encrypted DMs make it hard to create spam filters/tools to prevent harassment?
And here I've been tweeting at Musk to eliminate DMs