I was in a position to pay for my own post secondary education thanks to some very good summer jobs, and part time jobs through the year. My parents never questioned my marks. They knew, that I understood that this part was key to my future. Marks were never an issue for me.
I had friends in university who left. Sometimes poor marks, an over active social life was really an out for them. For some, university was not the right place. They were there because it was the thing to do or because their parents pushed them to it. Some have subsequently build very successful lives, careers, and in once case a very successful business. Post secondary was just not for them. They needed something else and were fortunate enough to find it. A few others returned...but when it was 'right' for them and they were truly ready for it.
We paid for both our childrens education. And today, if our son decided in his early 30's that he may return for additional post graduate work or to enable a second career, we would happily assist him financially. But we never considered paying their tuition entitled us to audit the results down to the end of term course marks. Nor do I think that they would have appreciated that incursion. If they do not get it by the third of fourth year of university then chances are it will take them a great deal longer, if ever.
Of course we were, and are, always there for them. But we do not expect them to live in our pockets. Nor do they.