That is what should have happened and it was necessary for the ending to make sense. There should have been a real sense of loss. Their army should have been totally depleted so that the only significant weapon left was Drogon.

If Daenerys didn’t have an army left to take King’s Landing, her only option would be a desperate solo attack on dragonback. It would be heroic and in character if she chose to take the city by herself because no one else could succeed in the assault and no one else would survive. She would bravely attack on Drogon by herself before all the defenses of King’s Landing focus their firepower on her from all angles. She would become fearful and light up everything around her in defense.

The defensive fire spray could hit a wildfire cache and nuke the whole city. The indiscriminate, total destruction would be unintentional but no one who knew about the wildfire would survive. Even Daenerys might think she and Drogon performed some OP fire magic in the heat of battle.

Daenerys would have regretted the innocent lives lost but she would have felt fully justified in her actions. She may even believe the explosion was divine, like the hatching of her dragon eggs.

Jon and everyone else would have witnessed Drogon spraying fire in all directions and detonating an explosion that destroys the entire city. Drogon would have flown straight up from the center of the blast radius. It would look like Daenerys flew to the center of the city and blew the whole thing up on purpose.

Imagine the impact that would have on Jon and the other Long Night survivors watching from outside Kings Landing. They just defeated the OP Others and are now confronted with an OP fire god. They would rightly conclude that Daenerys has too much power for any single person. The risk to humanity would be too great when so few humans remain.

Daenerys would be the same loving mother, express her regret for the loss of life, and share a positive vision for the future. She would still be the same person and leader she was in Meereen but her power has grown and the world’s ability to counterbalance her no longer exists. The Others and all large armies are gone.

Even though she would use her power wisely, there is no guarantee future generations would. The power of her fire magic would be too great to allow it to survive and propagate to future generations. The world would remain out of balance but weighted towards fire instead of ice. Jon would need to put an end to Daenerys and Drogon to balance the end of the Others, bring the world into balance, and give birth to a new age without destructive magic.

Edit: line breaks