It's time for a new tier of loot (Exotic's are too broad)
"Why would I ever use X weapon when I have Izanagi's Burden?"
That's a really good question. Another good question is simply:
"Why should I use X weapon when I have (insert next best thing after best weapon is nerfed)?
Some exotics are just bad. They're cool! But bad.
I propose in the distant future (D3 perhaps) there be a new tier of weapon called "Unique" which would be reserved for the exotics that are likely to under perform. Perhaps in the same way markets evolve... Supply vs demand (evolving world), etc. Exotics that are very rarely used should be moved to the Unique category, where they're no longer restricted to the "use only 1 exotic at a time" limitation, and could perhaps be added to Xur's inventory permanently due to overstock. This way, Bungie could continue making really creative and interesting gear without having to hold them up to the standards of weapons like Izanagi, and save the exotic name for weapons that are intended to be powerful.