(Also, again, my experience with 5e vastly differs from others'. As a player, I have found saves to be extremely binary--either they're trivial, or nearly unpassable. Often the latter. It's part of what makes combat, especially in the earliest levels, incredibly frustrating and demoralizing. I'm reminded of the "ghoul surprise" boondoggle back during the playtest.)
I don't know what Ghouls surprise you are talking about but most CR1 monsters have saves around 10 or 11 and those give you a rougly 50-50 chance at saving with a 10 in the stat. Imp poision - 10, Quasit poision/scare - 10, wyrmling brass dragon breath - 11, ghoul paralyzation -10 .....
Also with proficiency being +2 at low levels, I don't know how a save can go "tivial" to "impossible" the math just does not support that at low levels.
I would, personally, say that that means you're just seeing how niche the spell's effect is. As I very clearly said in a different part of the post you quoted, grease is not a good spell overall. But it's much better than shoving. Much as, for example, sleep is a LOVELY spell...at 1st level, and rapidly declines into "not worth the ink it's written with" territory by 5th level or so.
A sleep spell cast by a 5th level wizard using a 3rd level slot can on average put down a creature with 40.5 hp with no saving throw. By comparison a fireball cast against the same foe using the same 3rd level slot will do an average of 28 points of damage on a failed save or 14 on a passed save.
That is not to say sleep is as good as fireball, but it is hardly useless at higher levels.
It looks like you're correct. I had read the PHB rules--"If you're able to make multiple attacks with the Attack action, this attack replaces one of them," emphasis mine--as meaning you could use exactly one attack to shove, no matter how many attacks you might have. Assuming you allow Crawford tweets as official commentary on the rules, he has clarified that it is any number of attacks. But you still have to make multiple attempts at it--and you get at best four chances (for an extremely high-level fighter!) if you have four things adjacent to you. That's definitely not going to be any easier than getting four things in a 10-foot square!
In the vast majority of cases it will be substantially better for several reasons.
1. It is rare there are four enemies in a 10x10 space.
2. If there are four enemies it is even rarer still that you can get to all of them with a melee attacker after they have been greased, meaning this hurts you.
3. you can use the shove on the same turn you make the attacks, meaning initiative order is meaningless, you can shove and then attack the creature you shoved as opposed to a caster who casts grease and then has the enemy stand up before he gets attacked by anyone.
4. Shove works on enemies who are flying, levitating or climbing. Grease only works on those who are standing.
My math has, repeatedly, shown otherwise, so I'm curious how you reach this point.
And actual play will show that there almost never are 4 enemies aligned so as to all be knocked prone by grease and that even if there are you can't take advantage of it.