User blog comment:B214/Volume 23 discussion blog/@comment-78.48.65.206-20170426164843/@comment-79.25.167.4-20170427194252

If it was so, in EX then Sirzechs and the others sealed would have been freed by the barrier because then Issei, Vali, Shiva could join them to destroy 666 with one group attack. Issei to become one of the strongest, among the top 3-4, he will have to train for decades. Even if he grew up at incredible speed, it has only been because he kept to obtain new powers. But ExE would be the ultimate power up, then he will have to work for decades to perfect all his powers.

Right now Issei didn't even master CxC, imagine to master DxD, AxA and ExE how much time he will need, especially considering how much he will be busy in the future. Issei right now without using the "special forms" he developed, with his BxB alone is still far from being the strongest Sekiryuutei ever existed.