1. Advantages stack. 2. Yes, getting height is a core aspect of combat. We don't have any disagreement on this point. 3. I am skeptical that ranged attacks from high ground constitute such a huge percentage of rolls, even attack rolls, that all other rolls are "few and far between." This sounds like a personal playstyle quirk, not a fact of the game. 4. Be that as it may, having no need to take the high ground (if you don't need the extra advantage) is still an advantage. Being able to make other kinds of attacks besides ranged attacks from height with advantage is still an advantage. Negating the disadvantage from shooting up is still an advantage. Even if you built your entire character as an archer who maximizes advantage from height, even your entire party, Foresight would still be an awesome spell to have in your arsenal. 5. Nobody is building their characters around a 9th level spell.
Just compare these two things as if they were written out as abilities.
1. You gain advantage with ranged attacks from high ground. 2. You gain advantage on all attacks.
It's pretty easy to see which one is still better, and the fact that they are not mutually exclusive only makes both abilities even more powerful. With Foresight you can shoot from a tree into a cloud with no disadvantage. You can shoot your bow at an enemy in your face with no disadvantage. Without Foresight, disadvantage. Even at level 4 I am seeing enemies with 3-4 conditions stacking to determine advantage. Imagine at level 18 or whenever Foresight becomes available.
I just cannot understand why stacking bonuses negate each other in your mind.
Also for the record I tested the sneak attack mechanic and it is in-game, but unreliable, just as reported.