Arbitrary file access in the manner described is not a justification for classic - generally, for most use-cases the home and removable-media interfaces should provide access to most of the users files - can you provide any specific examples which would not be covered by these?
Also, could osync instead store this .osync file in another location to avoid this problem or use some other means to store this state so that it does not need arbitrary write access to all parts of the system?