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.
I'm not going to quote your whole post here just for space reasons, but I agree foresight is stronger and will be stronger into late game. The reason I believe it's comparable now is because most low level enemies really do just throw attacks at you, particularly the sort we spend most of our time fighting on a typical playthrough. There are better spells, but "In terms of combat we are getting a significant portion of a 9th level spell for the investment of a bonus action or some movement at level 2" is a good thing to point out, because it highlights how stupid the current system is. It's much easier for people to argue that a 4th level spell isn't really THAT strong (though those people would be wrong too).
I do agree backstab is worse just for how completely free it is, like you're just never going to not want to backstab someone. It also makes absolutely zero sense and breaks immersion pretty hard when you're doing this weird leapfrog thing with opponents where they apparently just don't care that you're walking around behind them. Flanking was dumb enough with it's conga lines as is, now we have combat look like some weird dance where people are just swinging around each other each round.
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.
Enemies absolutely don't have the advantage when it comes to shove, imo. They're always going to be the ones putting themselves near a ledge when they approach the high ground, unless you're letting them walk right up behind you for some reason. Having to use most of their movement to get to you massively limits their ability to position around you, while you have a full bar. The only ones who really can are some casters, but the AI is rarely smart enough to do much of anything with them from my experience.
Shoving is definitely weaker on most classes (though not to a reasonable amount, considering casters can shove minotaurs off cliffs), but high ground generally isn't that large, it's pretty easy to launch them off if you're positioned well. You are losing some damage by having a fighter bodyguard, but like I said, wizards being able to fully nova every fight and everyone having access to magic stuff makes that not a huge concern for the safety it provides (and the damage the fighter provides if someone does try to get up to you).
I think claiming it's more difficult than foresight when we literally just could not normally access foresight at all is a bit unfair, too. These things need to be judged in context, and we're not ever likely to have access to that spell. It is weaker and I'd be happy to use something like greater invisibility instead though. Probably a good example considering it's unlikely we'll be able to use 4th level spells regularly until pretty late game too. Though high ground is still stronger in most fights, considering it's unlimited (and free, like you said). This is on top of the normal positioning advantages of high ground, like being able to move back into cover, having much better sightlines, heavily limiting how enemies can access your location.
My argument about DC5 power word kill is to highlight another high level spell that similarly has most of its effects be pointless at our level (the number of enemies you'd need a 9th level spell to kill with ease in act 1 is zero), and the DC5 is to highlight that while technically you can fail, we'd all agree it would be a ludicrous reward for passing a check that easy. It's an intentional exaggeration of the point to show how silly the current situation is.
Sorry for awful formatting here by the way, not used to the quote system. Hope it all makes sense.
