See, this is why everyone thought you were being petty and argumentative
The problem with your insistence that the upper classare evul and never going to help the lower class falls flat here because the ruling class is dependent on influence, favors, and appearances to rise here.
Which means Nobless oblige even as propaganda has impact because the nobles have to maintain that pretence and can use that to attack others who don't go through the proper motions of supporting their subordinates. It's not simply a case of expecting the patricians to be virtuous, but altering the system to reward the (at least) appearance of certain virtues and letting them use that as leverage to undermine those who don't have even that much.
Keep in mind, we've already established the quality of one's subordinates reflects on ones own qualities. So naturally the King's closest subordinates should have quality subordinates, and etc. Just establish that one's holdings and those on them are also a reflection. Use said obligation not as an appeal to their better natures, but societal and cultural pressures that can open and close doors for them.
Sure, being petty and over taxing your holdings might give you money for gifts and bribes, but if it reflects poorly on you personally? Or is a vector your enemies could use to shame you? Than suddenly higher taxes and exploits become less attractive, because they're now as much a potentially liability than anything else.