Why would anyone assault King's Landing? Disease and hunger would have taken the city before long. The Unsullied were foreign invaders who put the capital of Westeros to the sword. They had no friends on the continent, following Dany's death. They basically had three choices:
- Negotiate safe passage out of Westeros.
- Try and fight their way out.
- Starve.
They went for option 1.
You realise Westeros has... no real armies left. The armies have all been in war, most of the men are dead. THere wasn't an army left in the seven kingdoms who could actually stand up to them and the Dothraki, also the latter could have simply ridden out and gone raiding. No army would be mobile enough to run them down. The Lannisters were the rulers of the 7 kingdoms(or 6), yet there were basically every army so weakened that they had to hire the Golden company to even have any kind of chance and they were taken out as well.
Every major fleet was destroyed, the only ones left a few from Iron Islands and the unsullied still had a bunch of ships.
The unsullied/dothraki's were the major force left. The Lannister army (the biggest one) was literally destroyed, both at Kings Landing, at the castle where unsullied were somewhat ambushed but they still killed loads of Lannisters. The golden company is gone, the early war with Robb took out huge numbers from the north vs almost anyone supporting the Lannisters.
The Unsullied and Dothraki could have simply said we are the power, accept that, start rebuilding Kings Landing, bring provisions or we'll march on one of your kingdoms and take what we need.
