That approach is more sensible for exlusive markets like utilities. If a better product exists they can go the way of Yahoo quickly. Now it could be argued they have too much inertia and splitting wouldn't actually help much - even so it raises another big question.
What regulation would even help? What is the actual explicit problem that could be addressed and is it even something addressable constitutionally? There is plenty of sloganeering and FUD but little thought into what would help from even limited to first order effects. Break up Amazon retail into five and the one with the name would win.
Also how does that qualify as a national security risk? I know the term is used as a thought terminating cliche for "shut up and obey" but I just don't see it in any way which respects democracy. People have a right to communicate with the outside world which allows data gathering and influence. Taking action to leave people completely "uninfluenced" would be totalitarian control.
The "unprecedented threat" reminds me of the anti-Catholic "if elected they would be a puppet of the Pope" - which shows utter historic ignorance given how kings interacted with the pope. He wasn't close to being in charge in a far more homogeneous and theocratic environment.