The estate doesn’t stay open - the loan is an asset of the estate and someone will inherit that asset, either because it was left to them specifically or because they inherit the remainder of the estate or because they are the people who inherit because there was no will. For example, my grandfather sold my mother his house and instead of getting a mortgage from a bank , she was to pay him $5000 a year until she paid the full amount. When my grandfather died, my uncles inherited the debt and she paid them until it was completely paid off.
But the executor ( or the heir) can’t demand immediate payment if the agreement was for 15 years and there are ten years left.