When people make great sacrifices for you, you are "in their debt"
They may not have the right to demand how they want to be paid, but you're indebted
Do I owe them? Yes! Can they demand exactly how they want to be paid? No!
Let's not cloak irresponsibility
If you're in a better position to "pay back" your parents, do so! To do nothing at all because you "don't owe them" is irresponsible