My guess is that someone in his situation, who has no criminal record but has made a credible threat, would probably be issued probation and required counseling. I doubt they would incarcerate someone who hasn't committed a crime, but would try to rehabilitate them.
That said, if you've got a history of unstable behavior and you threaten to shoot up a school, I have no problem taking away your right to purchase firearms. Play stupid games, win stupid prizes.
The only reason you're defending Nikolas Cruz's right to buy an AR-15 is because you and I both know gun control is coming, and it's a slippery slope

You're more worried about what's coming down the pipeline 6 months from now, a year from now, or 5 years from now.
I'd argue that if you really want to preserve as much of your status quo as possible, you might want to do everything possible to prevent the next Nikolas Cruz from purchasing an AR-15. The days of "shall not be infringed" are over; declining gun ownership guarantees infringement will happen, but the number of mass shootings going forward will dictate how much infringement will happen.