Were you excited when Tera Gold first introduced its current crop of riding dragons? Maybe. But now those dragons are boring and dumb, and thus your interest are only able to be retained with the introduction of recent dragons that happen to be much cooler than the existing, boring dragons. That’s why the overall game is bringing out a different crop of bone dragons and telling all of the earlier versions of dragons to consider a hike and don't come back.
The net result's that all in the cash shop strategies of acquiring the current crop of dragons Tera Items will probably be retired on April 20th, that is (not coincidentally) in the event the new round of dragons will likely be introduced. You will still be able to get the old dragons inside the Dragon Scale or Dragon Token shop, but that will likely be the only way. So it’s out with the previous, together with the new, of course, if you really want those existing dragons, you must work on that before next Thursday.