User blog comment:B214/High School DxD EX/@comment-50.139.232.230-20151108030319/@comment-50.139.232.230-20151108225643

As a heads-up, the artificial Boosted Gear that appeared in the story is meant for Ix's use and Ix ONLY, since it is built to fit his fighting style. What Airi is saying is implied to make him cheer up, but it doesn't seem to be working. The implication is, yes, Ix is the one and the only chosen to be Issei's true successor, not Airi or Kurenai or others. Even the other siblings already acknowledged that.

Thirty years from the current timeline the Grigori has perfected the artificial Sacred Gear system to the point of creating artificial Longinus. Ix's gauntlet is one of the prototypes, despite holding only one fifth of power of the original. Plus, they also possibly have perfected the Sacred Gear extraction technique to the point that they can prevent the lethal effects of Longinus extraction.

Remember that Ddraig only exists in spirit form in the present, so him moving from time to time to other Sacred Gears like Divine Dividing does not restrain Issei very much, only his Boosted Gear abilities (well, his consciousness, but it also counts), so in the future, he can possibly move freely from Issei's BG to Ix's and back without much hassle (but this depends on how far the Grigori developed Ix's gauntlet and Ddraig's willingness to take that path). They already did that much with Airi, making Fafnir, who is still alive and kicking, easily go in and out of her artificial Sacred Gear without straining her body too much or breaking the Sacred Gear.

The Counter-Balance phenomenon is tied to a Sacred Gear theory/clausal that Azazel was trying to propose in the future, namely "Selective Restriction". In short, only the chosen and the worthy may wield Sacred Gears, regardless whether they are genuine or artificial. The reason? The less hassle people can enjoy in the future of activating their Sacred Gear without so much of conditioning, which result in many cases of normal Balance-Breaker going wild, and Counter-Balance concept is developed from there.