I think there are two components here:
1)The halachic requirement
2)The relationship itself
Even if a parent is mochel on his kavod I definitely think that a person is misaken his middos by standing up for the parent.
Is kibud av a din on me or on my father? Meaning, if my father is mochel on his kavod do I no longer have a mitzvah of kibud av? Or do I still have a mitzvah of kibud av, but I won’t be oyver by not standing up for him?
I would think that it is the second. The nature of a parent/older person etc. is intrinsic to the parent/older person vis a vis the child/youngster. You can’t separate the ‘parent’ from the ‘person’ from the aspect of the child, there is only the aspect of ‘parent’. Whatever the parent does with respect to the child has to always be approached with the perception of ‘parent’.
So too, age is as inseparable from a person as the color of his eyes. The relationship between a youngster and an older person has to always reflect that.
They are all people, but one of them is wearing an immovable ‘crown’ on his head and the other isn’t. By virtue of that crown, he deserves honor.
From the aspect of the halacha and the parent/older persons’ own tikkun hamidos, I would imagine they should not feel slighted.
But from the aspect of the nature of the relationship it is understandable and I would argue their right to feel slighted.
Trumah is not comparable because you are not required to give Terumah to a specific kohen, he can’t mun you for it. You do have to give kavod to a specific parent.
The mitzvah of tefillin is not for kavod hatefillin but for the toeles the person gets for wearing them. Kibud av is for the kavod of the father and their is also toeles to practicing it.