In the case of Eldritch Spear, the player invested valuable character resources into the ability to push people off of cliffs. What's punitive is when you negate that by allowing his target a saving throw. It's an extremely situational benefit, and you've just taken it away because your monster isn't smart enough to move the fight to a safer spot, but you think it's somehow cheap or anticlimactic to let the players use gravity (and the monster's stupidity) to their advantage.
The yakety-sax scenario is only going to be a thing when the combatants on either side go into such a battle unprepared. If your party prepares spells like feather fall and levitation, and if you do everything in your power to avoid getting near the edge, your chances of not falling increase drastically.
If you don't take those precautions, you deserve every bit of yakety sax you get, with Benny Hill dancing on your grave when you hit bottom.
I agree with you, but I do think that cinematic fights like at the top of waterfalls or on rocks in the river, etc. will be avoided by players if a single simple to hit takes them out of combat or kills them. That's a little too similar to save or suck (although it is a to hit roll, not a save).