how many kilometers in these 5 years?
the problem is when you ride, not when you are parked, especially in crete where it is dry, you have a lot of dust in the air and the engine simply sucks it up.
the first place to check wear is the piston rings GAP.... not the cylinder wall 
also, the dust in the air is very fine, will not make scratches in the wall, but it slowly acts as good fine sanding paper, no wonder your cylinder "looks like new"
I dont believe that what i am saying will make you change your mind and run an airbox and filter, but so just that you know there is no free lunch, airboxes and air filters exists for a reason....
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