User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-67.176.158.104-20140207023318/@comment-1359322-20140207114557

The Juggernaut Drives devours the life of the users, it isn't related to how long the possessor can live, the Juggernaut Drive simply uses the life energy of the possessor as a fuel, even a Dragon's life energy is limited, they are living beings, they need to replenish their energy and all too. So in that sense, Issei if he didn't overcome the Juggernaut Drive's curse would still die even with a Dragon's body.