Alucard in fact hears none of the conversation. His shoulders are up around his ears, too defensive to truly listen in. D has in fact called him out. The words ring in his ears, And when the Belmonts are dead, when all you have is empty rooms and forgotten legacy...?

Oh, there was more to it than that. Alucard knows there was more to it than that, but he can hardly make himself care. He has thought so many times about what he'd do when no Belmonts remained, and every time he dealt with the thought anew, he ran from it.

He'll be nothing. Easier to go to sleep again and be awakened if needed. And if he is never needed again, then so be it. He knows the other option, the path of his father, that is unacceptable. So it cannot be contemplated at all.