GMs tend to do that already when things are truly supplanted. There's just that many different roles to play.
Eh? Is there an Arms Race where this is an explicitly mechanic and not just something GMs do because they've gone over the word count on a post? If there is I wasn't aware and I retract what I said.
Otherwise:
I'm not sure if you understood my suggestion? I know that GMs already push older designs into a "legacy" or "obsolete" category when they interpret them to be overshadowed by another weapon, but they do that to save space in equipment posts and because they can't write about every single weapon in battle reports, it's not an action the players make and no one is rewarded for it, that's just bookkeeping. I'm saying to put the responsibility on the players to decide when something is obsolete and reward that action with a bonus to a design or revision roll.