Part Of the Furniture British informal
Something or someone has been somewhere and become so familiar that it is hard for you to accept that place without them.
Amy has been working here for so long that it's like part of the furniture.
Tom knows everyone in the company because he's like part of the furniture.
This idiom alludes to the fact that the furniture in a building, such as chairs, tables, and desks, tends to remain in the same place and never be moved, so that people who work or visit the building often do not really notice it, just as the decorations or paintings on the walls do not notice them. Used since the latter half of the 1900s, at least.
Difficult to choose between two alternatives.
I am always caught between two stools when choosing what to buy at a bookstore.