That’ll only work if they stay in the dungeon. If they leave, they could earn the peoples trust and overthrow him.
That might not work either. If word were to spread that the former Grand Mage, the former Hero, the Queen, and the Princess all banded together, even a hint of any activity against the Emperor will make their political supporters flock to the Dungeon. The percentage of the Kingdom's soldiers under them all become the armies of the new "Demon Lord".
I'm getting ahead of myself with this, but if it goes in that direction, they will bring what will likely be a tremendous chunk of the economy, all the money in circulation which they control, goods, services, food, trade, banking...the Guilds are gonna be in a state of panic! Then there're their trading partners outside the country, and X number of other nations who also have a bone to pick with the Emperor, who will at least send representatives to ask WTF happened, maybe even ally themselves with the Dungeon eventually. The country has essentially been pushed into a corner, and ownership is threatening to change. And all it took was for the Queen and her daughter to run away from home, and into the arms of her biggest simp and his best friend.