1. Advantage does stack. Ludicrously, somebody quoted the specific rule explaining how they stack while simultaneously claiming that they don't stack. That is either deliberate absurdity or the very definition of a bad faith argument.
2. It is not contradictory to note that getting advantage is a defining feature of combat, while also disputing that all other combat rolls are "few and far between."
2a. Somebody said that my interpretation of the term "few and far between" as meaning less than 50% was too strict. If non-ranged, non-high ground attacks rolls constitute more than 50% of your attacks, then they are not "few and far between." If something happens more than once out of two times, it is common and frequent.
2b. If it is unfair for me to assume that "few and far between" refers to 50% of rolls or less, then it is also reasonable to conclude that Foresight will still provide advantage on more than 50% of attacks, making it incredibly awesome even in a world with high ground continues to provide advantage.
I think we fundamentally disagree on how DnD is played. I absolutely love that a shove is more powerful than any spell in the right circumstances, something that is also complained about elsewhere in this thread. To my mind, that is as it should be. Shove and the high ground advantage both simulate the creativity that I expect from my game and which the DMG encourages. Situational bonuses like the high ground advantage or a well-timed shove should be replete in DnD, far MORE common than a 9th level spell. As far as I'm concerned both are a pale imitation of what the tabletop offers but far better than nothing.
If you don't like the mechanic, fine, but your complaints that it is totes way more powerful than a 9th level spell that guarantees you sneak attack on every melee attack or massively extends your ranged weapon reach, both beneficial effects of Foresight, is simply ludicrous. If you have to resort to that level of hyperbole to make a point, it makes it seem like you don't really have one.