The disbelievers (who ask for a miracle) would not have believed, even if this Qur’an had made the mountains to move, the earth to open up or the dead to speak (as it happened in the case of the previous prophets.) God is in control of everything. It is too soon for the believer to be disappointed (that an undeniable miracle is not being sent.) If God wills, He can guide all the people (which, by the way, happened in their own life time when after the fall of Mecca, disbelievers embraced Islam by groups.) The disbelievers will continuously suffer directly (as it happened battle after battle) or indirectly in account of their own mischief. The Lord’s promise will be fulfilled; He never fails to deliver.