I'm a strong proponent of free software, but I do see your point, that there's safety for non-technical people in a walled garden, as long as you trust the gardener.
I'm wondering if the concept of free-software and a walled garden are really that incompatible. Let's say the garden has a gate, that a user has to go through a clear process in order to allow to install non-recommended software.
This would give a similar level of protection for non-technical people, but will still be free-software, and allow technical users to install whatever they want.
One effect would be that, if the gardeners decide not to recommend some popular software, then many users would be inclined to disable the safety features, thus reducing overall safety, so there will be a penalty if the gardeners are too strict.