All my life since I have grown up I have heard no matter what you cannot miss your wajib namaz. Even if you’re sick you can sit and offer your prayers or even while lying down but missing it is not an option unless you are unconscious. But then I see people offering qaza namaz for the dead person. I’m unable to understand this concept. It’s contradicting and means you intentionally don’t pray all your life and your kids or someone else will pay for your qaza namaz and fasts and Allah will accept. Then this statement that you can’t miss your namaz is asked on which context when it can be offered by someone else when you’re dead. It makes me think I should also not do it as I’ll ask someone else to do it for me after I’m dead.

If you intentionally left your prayers or fasts, then, your oldest son is not responsible to make qaza.
He is only responsible, if you missed prayers due to ignorance, such as you did not pray while you were sick, thinking that you have an excuse, or if your prayers were void, because your wudou was void…etc.
Some people do not perform wudou correctly, and they realize this after years, then they start to perform qaza, but they die before completing it.