In my opinion, none of them should progress.
Leave the past where it belongs, in the past.
Of course this is 100% dependent upon Bungie being able to make suitable replacements and that would take more effort than just giving different upgrade materials to make guns like Gjallarhorn viable for the length of the franchise.
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Well when you distinguish a weapon like they have there exotic weapons you almost have no choice but to allow them to continue. There is not enough exotics in game to do away with them all.
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If Bungie was able to make suitable replacements, yes, yes there would be. Of course that would require them making about 4 different exotic weapons per weapon class to do it. Hell of a lot more work than just allowing upgrading previous guns.