Pretty sure that's core to many of the complaints.upupgraded exotics you may have in your vault are oddly valuable as a result. the upgrade exchange will take them from 260-302, the largest damage just that any upgrade process can even do right now. presumably, after the exotic update, this is also an opportunity for other people to main the reworked exotics for fun and a "cheap" upgrade.
thinking of stuff for a guide on how to navigate all this stuff, if there would be interest. need some more info, but pretty soon.
At least when you dismantle a fully leveled (not even upgraded) legendary you get some small bonus out of it. A little more energy and a little more telemetry.
How about when Xurring a fully upgraded exotic you get an eight hour buff? For the next eight hours you get Rahool's Trollface equipped. It would be similar to the Halloween pumpkin head. Also all of your gestures are replaced and instead your character calls itself an idiot for wasting so many resources upgrading exotic weapons in the first place.