My magnolia has grown to be about 25 feet high and I'd like to reduce its size before the top snaps. A tree expert told me topping would kill the plant. I was concerned about causing bushy growth but would trimming really kill it?
You should not 'top' your magnolia. It is stressful for trees and is not recommended.
Unless it was poorly sited in your landscape, it shouldn't need any reduction in height, especially at 20 feet tall. They naturally grow tall and strong (up to 60-80 feet) without any problems.