I agree foresight is better overall (particularly late game and out of combat), but I'm not convinced that makes the comparison unfair, particularly in a lot of early fights where all you have are really attack rolls, and when we are just talking about combat applications. There have been better, longer discussions about all the weaker abilities that high ground and back stab completely invalidate too, but I don't think it would contribute much to bring them all up here when foresight is just as useful a comparison.
I don't think Foresight is an apt comparison because the only part of the spell that high-ground conditionally mimics is providing adv/disadv. for range attacks to enemies below. Even if we're just talking about combat applications - getting advantage to ALL saving throws an unlimited amount of times with no concentration is something insanely rare (maybe even only granted Foresight spell, I'll need to confirm that). That on its own is arguably more valuable than attack advantage and defense disadvantage - since we've seen how many scenarios can grant that.
Also, the benefits of Foresight is basically impossible to counter (at best it can be evened out some of it by imposing adv/disadv). An ancient dragon can fly up and attack your face and you will have adv/disadv without having to worry about losing it through concentration issues. It can grapple you, breath fire on you, and you'd make all those ability checks and saving throws at advantage. It simply cannot be reliably taken away from you short of a level 9 dispel magic.
It makes for a nice soundbite to be able to say "lol Larian is giving everyone free level 9 spells with high ground", but I honestly think it just weakens the strong logical arguments I've seen here.
Greater invisibility, shadow of moil, darkness + devil's sight are far better comparisons because those all provide similar effects with comparable weaknesses. E.g. you can make the argument that maintaining concentration and high ground is roughly the same level of difficulty, and that will better illustrate how overpowered high ground is, because now you're comparing a 4th level spell slot to a conditional but resource free effect.
Backstab is its own problem (and I think worst than height advantage) because it essentially just turns on Reckless Attack for everyone.
Also, it's not at all difficult to maintain high ground in most fights, particularly with how strong shove is. Enemy uses an ability to get to where you are, you just shove them off the cliff or ledge, it actually makes them trying to get to you a worse move than just trying to find somewhere safe-ish and shooting from the ground. You can regularly just park your whole team on high ground and be untouchable, wizards being able to nova every single fight means your damage isn't really a big concern, and there's zero risk to it because your melee characters can launch people who try to force you off.
You can still fail to maintain it, but you have to screw up in so many different ways for that to be the case, hence my comparison to a DC5 check. Comparison to it being like us getting power word kill for a persuasion check is similarly valid too, because most of the power of that spell isn't actually relevant here either. You could kill enemies instantly with disintegrate too, but I think it makes a point to use a stronger one.
IMO shoving is somewhat of a wash when it comes to maintaining high ground. The pursuer will technically always get to shove first (if they dashed in instead of jumped) since they are usually the one closing in on their turn. The AI isn't incredibly tactical, but I have seen it try that from time to time.
People who can take advantage of high-ground - DEX martials or casters - are unlikely to be good at shoving due to a low Athletics score. To your point, you can plant a dedicated STR based linebacker there just to do your shoving. However, I'd argue parking an entire character there just to maintain high ground is a cost in terms of actions and damage output (especially if you have STR based fighters that have poor range attack and can be doing something far more productive with their actions). As a side note, it's rare, but bunching up on a cliff does risk your party getting launched by a Misty Step + Thunderwave combo. The Duergar Wizards in the Underdark do have a tendency to try to launch people off quite a bit (by the docks and especially on the boat).
Overall, it's not necessarily difficult to maintain high ground, but it does consume resources in its own way (which you have to factor into the calculation for what high ground advantage is worth). My point is that compared to Foresight (which just gives it to you), it is far more difficult.
I'm not too sure what you mean by a DC5 powerword kill, so I'm not going to comment on that.