Co parenting

So have you ever notice when couples have a baby together and end up splitting, they always say “I only have to deal with them for so and so years” “I only have to communicate with them until the child(ren) are 18 years old” ? But in reality you will be most likely dealing with the person you had a child with for the rest of your life.. I get why in a way they say that because if you co parent then yes you will need to communicate and make parental decisions and do drop offs until the child is 18 years old but I would think that they would still have to even after that because once the child graduations college, gets married, has a child of their own, or even if there was a major life changing event (rather it be good or bad), obviously they would want both parents there (I mean if they had a bad relationship with one or both of their parent then that would be different). I just feel like if me and my fiancé were to spilt up with our son being 8 months old, neither one of us would say we only have to deal with each other until our son is 18 because we knew from the beginning rather we would be together or not that we would be in each other’s life for the rest of our lives or as long as our son is here and still living. Am I missing something to why they say that? I am not criticizing anyone or telling anyone how they should communicate with their fathers/mothers children but you never really stop being a parent to your children and some big things in life, you both would want to be there for your child so even if the child is an adult, you would want to act cordial for the sake of the child because you don’t want to make things awkward for them or yourself and the only way not to make things weird is by being friendly, not saying to have a full on conversation but saying “hello” or “how are you” would be best... or maybe not.