Hard to argue with results, and most military approaches that survived did because the people using them did.
More to the point, uneven terrain is an obstacle, and digging ditches to build a berm is a logistically cheap and technologically uncomplicated way to produce it.
They didn't fully hand it to Linux yet. We still have to earn that. Ideological appeal / privacy concern alone isn't enough for many people if the jump seems too scary, particularly if it feels like a one-directional leap of faith. What if they don't like it on the other side? Better the devil you know...
We need to build bridges, in both directions: help and encourage people to switch to Linux, but also promise them help to get back, basically an "out" if they don't like it. I see plenty of guides for migrating to Linux, but how about getting back to Windows?
It's okay not to like Linux, it's okay to be scared or apprehensive, and it's okay to get cold feet and return to the familiar. Maybe some time in the future they'll try again.