I live on Long Island, NY. It's the home of the "Gold Coast."

In northern Nassau County, there's all these 100+-year-old mansions, on a gazillion acres.

Even though they are often owned by people that can afford the upkeep ("old money"), they tend to get sold or donated to the county or state.

There's a few great museums:

https://www.oldwestburygardens.org

https://plantingfields.org

https://nassaumuseum.org

and a few in Suffolk, as well:

https://bayardcuttingarboretum.com

https://www.vanderbiltmuseum.org

They are a pretty big pain to keep up. There's also a couple that have been abandoned, because the owners couldn't give them away.

They cost a lot of money to maintain and you cannot just let them decay because they come with an obligation that you maintain them. This means not only a vague general obligation but the monument authority will make specific specifications what has to be done and what can't be done. You'll have to get approval for every modification down to little details like the very specific type of window frame and such things. In some areas these obligations can also include that you open your premises for the general public once a year for the National Heritage Day.