I did a post about this. My idea...let us use old Raid weapons as crafting materials to create "adept" versions of new legendary weapons. It would make raids useful again, diversify weapons and be awesome.
Example: Oversoul+new legendary pulse= Arc legendary pulse.
We do not know what is coming down the pipe besides some vague youtube stuff. We have to wait and see. It appears that weapons will start being similar to thorn in that it will take steps to get good stuff.
On Reddit Bungie specifically said not to scrap you stuff just yet, that there is a use for them, and new GI article let slip a comment about using old weapons to upgrade new ones. So it may just be like an xp boost on the new stuff. But I very much hope that it is this, it makes so much more sense. It would be a reason for new players and old to make multiple runs at older content. Otherwise even new players aren't going to care about doing VOG hard just at a 5% chance or less at a Vex. They'll do what I did with alt toons, skip right over content for the higher level/better reward stuff.