If they make new Legendary weapons...
Its not IF anymore, they ARE making new legendaries. Its probably a pretty good chance that the gift format will stay, but the precursor will change to an earned precursor instead of an RNG precursor. They said when they bring in precursor crafting that they don’t want to wreck the current model. So that more or less leaves them with the new legendaries starting off with the new crafted precursors.
You should keep the mats, but don’t make any gifts. There is a chance they won’t use them all, but it is more likely they will use different gifts. Mastery, though, will probably stay the same.
Everything we have is conjecture, obviously. If they came out and said “This item will be used to make the new legendaries” it would break the economy of everyone throwing themselves over the dwindling supplies in anticipation.
Any new legendaries will probably be made differently and feature the mystical precursor crafting or an entirely new method. The current legendaries and their precursors will probably remain untouched, since people will complain either way.
New Legendaries will probably require 500 crafts T7