You paid for it, you got to use them all...You'll pay for Taken King and you'll get to use them all. Guaranteed there are new exotics that will be similar to the ones being left behind, becuase those being left behind are pretty average at best. I never used half of them because there was always a top tier of exotics that beat everything else out. I regularly only use Helm of Saint-14 and Helm of Inmost Light...because the rest of the Titan exotics are trash, except maybe Feedback Fences. Ghorn, TLord, Truth are the only exotics that really get any use for me...because the rest are just underwhelming and got nerfed to oblivion. Tons of them are just plain Meh....and I wouldn't be surprised if they get redesigned and re-added in the future. I mean there were a ton of empty spaces where new ones can be added, it'll happen. Legendaries from the raids were often hands and fists above the exotics and way more useful.. They'll all be useable in the vanilla stuff too, if you wanna go back and do Patrols, take a Year 1 with you....or an Old Raid or something else, there will still be plenty of times to use them. Its not like all the other base content is disappearing.