I don’t think I could eat the same thing every day unless I was in serious hunker down/survival mode.
Well, I guess if you had dangerously high cholesterol, that might constitute ‘survival mode’.
But I’m sure there are other things - like beans - that can lower cholesterol. It seems to be the soluble fiber that does it.
https://www.lipid.org/sites/default/files/adding_soluble_fiber_final_0.pdf
You can when you have to due to serious food allergies that restrict what you can eat.
And yes, living on oatmeal is GREAT for cholesterol.