Levelling exotics is the most arduous task in the game outside of levelling a character.
There are many ways they could have forced us to earn he new upgrades without resetting the ones we already earnt.
We should be able to use the weapons we spent hours upgrading to earn the next tier, not be forced to repeat the exact_same_grind.
This is not fun design.
Seems to me from a technical implementation standpoint, all they would have to do is:
1. Create "Xurbished" versions of all the exotics. Same bubbles in the progression tree, but Xurbish bubbles unlock with much lower XP (say 50% less?) versus the non-Xurbish version.
2. Xurbished version can only be acquired from Xur in exchange for a FULLY UPGRADED non-xurbish version (meaning all bubbles unlocked via XP, but not necessarily purchased).
This would solve a couple of issues including players feeling like all their XP used to upgrade the "old" version is completely devalued.
The NEW non-Xurbished version would simply have the NEW attack values with the same XP progression tree as the OLD non-Xurbished version.