If you sell, then say goodbye to your city. If you don't sell, then you don't have this capital, and you still live in a very expensive place.
You can mortgage it for a lot of capital, but this is a poisoned chalice.
Other means to actually realise any of that wealth is to try your luck and speculate downwards, which is extremely risky if that's your only big asset.
Lots of people are "feeling rich" without being realistically able to realise any of this wealth, since they cannot really move away from their city. This is fictitious wealth that constitutes a massive burden to the city and endangers its competitive place in the future.
It will pop sooner or later and its likely to cause massive social problems. It's causing them already even before that happened.