if it were zero sum then how come middle class people nowadays live better lives than kings 500 years ago?
How many middle class people have the God-given divine right to rule and demand the sons of every nobleman to fight to keep and expand their power?
How many middle class people literally own a country and can dictate what happens in it from the top down? How many vineyards do middle class people own? How many garden hermits[1] do they keep on their estates simply for their ambience and kitsch?
The second thing you're referring to is power. Power I admit can be zero sum but most people are not ambitious in that way, they just want material wealth not political power.
Similarly, spending $10 at McDonald's is not the same thing as having teams of servants tending to your every whim every waking moment until you die.
If you can't see that clear difference, I don't think you're approaching this in good faith. You shouldn't have to stretch the truth this much to make your point if it's actually true and you know it.
I have air conditioning.
I have running water and a toilet.
I have antibiotics. And all sorts of other medicine that actually works.
When I have to get an operation done I can go under anesthesia rather than it being literal torture. And I won’t be killed by an infection resulting from the tiniest cut.
I have literally endless entertainment. More than I can ever consume in a lifetime.
I have access to just about any piece of information that has ever existed and been made public.
I don’t have to worry about being killed by an uprising.
I can go on.
I find Spanish castels from 1500-1600 to be pleasant enough temperature wise even when it's +35C outside.
You're comparing a ten dollar meal from McDonald's to a cooked meal by a team of professionals. Your comparison completely ignores the fact that in modern society you can absolutely have a meal cooked by a team of professionals or you can have McDonald's and you can decide moment to moment what you want, and all of it requires less outlay from you than it did 500 years ago, so it's available to practically everyone, not just the most wealthy.
500 years ago puts us in 1525, which is still in the transitional period of Medieval kingdoms becoming actual states. The kind of absolutism you're thinking about is still about 100-150 years away from coming to fruition. Medieval kings were almost entirely reliant on their vassals to be able to marshal any power whatsoever: if an important vassal decides he doesn't want to fight in the king's war, well, the king can't exactly force him (see e.g. the Burgundians in the Hundred Years' War--and this involves the most cohesive European monarchy!). You're starting to see efforts by 1525 to reduce the power of nobles to act independent of their liege, but they're certainly not there yet.
But keep in mind the other luxuries people today enjoy that the kings of 500 years ago can't. The ability to see all of your children survive until, well, your own death. The ability to eat, say, fresh strawberries in the height of winter. The ability to know your wife isn't going to die giving birth to another kid. Hell, pretty good odds your bed is larger than a king's bed from 500 years ago, and more comfortable too.