Exaclibur prime will return and do have some reason on why he would.
1. Exaclibur is the main icon for Warframe, so they would not keep one of the icons of the game locked.
2. With PvP coming out soon, a lot of people would be claiming he would have an evantage over players with the non primed one. He is one of 3 stat changed Prime frames.
3. It could have been a timed exclusived. From a marketing standpoint it would make sense, get people to buy it now and re release later. It may only be a Pime Access.
4. It counts for mastery rank.
5. With Warframe now on PS4 and going to Xbox One, them telling thier new users "here is a frame you will never see" makes for bad business. So when they release on PS4 and Xbox One, PC player would go mad and so they would re release on there as well.
6. Most people we hear from with in DE is community managers whos job it is to make you feel good. A good 90% of them don't know what goes on in the marketing room or even what the heads have to say. most community managers are out of the loop as seen from miss infor from them.
7. He is a starter frame. it was said many times they want all starter frames primed so with him being locked they really can't do that.