Kinder reader Inflection wanted
The OPs problem was enemies being knocked off cliffs to there death. That being the case the damage you dealt is irrelevant since both result in 0 hit points.I think the biggest issue is that while a fighter can push up to 5 times with no resources, their shoves are not also inflicting damage at the same time, and they have to be in melee range, and the target has to be no more than one size larger, and they have to use their movement if they want to shove more than 5' distance.
That's a lot of extra conditions.
Also, as others have pointed out, usually an attack rider allows a saving throw to avoid it, even though there was already an attack roll made to inflict damage.
I don't have a strong opinion one way or the other on repelling blast as I haven't seen it in play yet (no warlock lovers in my crowd, I guess), but even without specific scenarios I can see that some might be inclined to take it down a notch simply to bring it more in line with other cantrips and abilities.
Nothing wrong with that, even if you or I or anyone else happens to disagree.
Lightning Lure, would be the closet example of what your talking about, however my point is its not the spell Eldritch blast that grants the "push" effect. Its the Repelling Blast eldritch invocation. As an invocation that requires you to invest in a spell to use it, its kind of a double the cost for a lesser effect ability. As I said, I used it... Lance of Lethargy is simply better unless your GM is running a campaign that takes places on the cliff of a mountain. Even if your fighting Storm Giants on floating city most of the fighting will be in buildings, on level areas in the middle (roads, court yards, etc) and when you do find some one on a ledge they are likely facing out toward you and repelling blast would push them to safety not off the ledge. So they added Grasp of Hadar which does that.... but then it furthers my point because the warlock taking a spell and now two Eldritch invocations is investing a lot of core class recourses for a rare event where they look cool.
Again if a GM feels the need to nerf a player, do what you have to do. I am just advocating that before you ever nerf a spell or ability that is clearly working as intended form a player who wants it for that intent, consider if its really all that bad. If your not putting enemies on ledges, its already restricted to crowd control. When I used if for that all I did was annoy melee characters in my party.