Is this something to worry about?
The inner chain stays age rubbed down to the bare metal on both sides. This can only happen when the mud buildup on the rear wheel rim is enough.
As the frame material is aluminium, corrosion won’t be a problem. The abrasion has gone through the paint, under-coat and some metal layer.
Besides, the tyres are now new. Previously, punctures were becoming frequent on the rear and grip quite poor.
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Schwalbe Rocket Ron types and the same kind of thing as the original pair. Perhaps fewer punctures also.