Bungie, in the future stop the random drop for the exotic raids. Or otherwise, insert a logarithm that increases the possibility of obtaining the weapon, each time the raid is completed without obtaining the weapon ... like in the nightfalls. Bungie please listen to us.
Thing is though, systems like that might favour the player but they're disastrous for the developer. They're job is to keep you playing, if everyone got a guaranteed exotic on their tenth run, barely anyone would do 12. This is happening right now with the menagerie due to the glitch. If they ever did implement a system, it would really only kick in after many many clears.