Mosaic Estimated Rental Fee fixed?

What is the reason the Estimated Rental Fee of a mosaic is fixed? I use wallet 1.0.4 and the fee is always 50XYM (estimated)

A mosaic with a duration of 3 days costs the same as unlimited duration. Why not the same structure as namespaces?

PS: unlimited is 10 years?

NEM/NIS1 or Symbol?


@gimre could you explain please?