I really hate food waste and would eat it. I'm fine with 90% of the time it being fine. I'm really glad you survived your open oats encounter!
Remember what they did before refrigeration (besides get sick). They had no option but to leave it on the hearth or stove. Ideally, there wouldn't be any leftovers, but if there were, breakfast was planned.
Pease porridge hot, pease porridge cold,
Pease porridge in the pot, nine days old;
Some like it hot, some like it cold,
Some like it in the pot, nine days old.