Another thing while looking at possible spells to scale:
During my search I stumbled upon erupting earth. My math reaches another conclusion then yours: The expectancy value of the d12 is 6.5. Calculating with that I get 19.5 base damage scaling at 6.5 dmg per level. The fireball scales at 3.5 damage per level. So the difference shrinks by three points of damage per extra level used. This means they deal the same damage when using a 6th level slot (which is quite high).
TheBigHouse has a wizard guide that does a great job of comparing the scaling of blasts. You can find it
HERE.
I've had some conversations with him, specifically regarding Erupting Earth's scaling value, and scaling of blast spells in general. My view is that fireball is a very good 3rd level spell and a terrible 6th level spell. Doing 39 average damage (on a failed save) with a 6th level spell slot is abysmal. Being as good as fireball cast with a 6th level slot is not good, it's not even so-so.
Essentially, I would argue there is no spell slot where Erupting Earth isn't awful, even though with high level slots it becomes a bit less awful than fireball becomes.
To understand how poorly these spells scale - let's take a trip right to 9th level and compare them with a 9th level blast. Fireball will provide you with 14d6 (8d6+6d6) or 49 damage on a failed save. Erupting earth will provide you with 9d12 (3d12+6d12) or 58.5 damage on a failed save. Meteor swarm will provide you with 40d6 or 140 damage on a failed save, this doesn't even take into account the larger area, the longer range, or that it will ignore a globe of invulnerability.
TL/DR: Never cast Erupting Earth or Fireball with a 6+ level slot. Blasts don't scale well.
I would argue though that even at lower levels the trade off is worth it. 1. Erupting Earth deals bludgeoning damage which is generally a better damage type and 2. It turns the area into difficult terrain, adding some utility.
It's no secret I'm a fan of battlefield control - but turning a small area into difficult terrain is almost trivial. As for resistances/immunities - you are correct that bludgeoning is (significantly) less common, however, I don't agree with the conclusions you've drawn from that. Keep in mind that at 3rd level (because we aren't using higher level slots, because of terrible scaling), the average damage of a fireball is 28 (pretty good), and the average damage of Erupting earth is 19.5 (pretty bad).
When I hit something with a fireball, and discover it has resistance (I would never intentionally case a fireball at something I knew had fire resistance), I am disappointed, because now my fireball damage drops into the mid-teens. That's shameful damage for fireball. For Erupting earth, that's only slightly below average, and you can expect underwhelming damage with every casting, regardless of the enemy's resistances. That's a kind of consistency I don't find compelling.
TL/DR: Erupting Earth's damage is consistently bad, so it doesn't matter if it is seldom resisted. Making a tiny area difficult terrain is not particularly useful.
According to this chart, which seems to be mostly correct (baring some minor rounding errors), Erupting earth seems to be the overall best scaling AoE spell at levels 5-8. It is only outdone by Vitriolic Sphere, which does it's damage over two rounds:
https://docs.google.com/spreadsheet...lFDBoeiiLsuzq94fBjwkoHKm18/edit#gid=124665534
Imagine you go to a fish market and there is a shrimp vendor there, you like shrimp, so go to take a look what he has. The smell is overwhelming, and the shrimp vendor explains that this batch is all pretty old, and probably rotting a bit. He holds out a shrimp to you and explains that this is the "best" of all the expired rotting shrimp he has, as it's expired by less time than the rest. Do you eat it?
This is why that chart is not particularly meaningful. Blast spells universally scale poorly. Finding the one that scales the least poorly doesn't mean it scales well. It scales poorly, just like all blast spells do. That it scales a bit less terribly than other blast spells is kind of irrelevant.
Finally, I would add that there is a reason I don't rate any blast spells between levels 4-8 particularly well, as blast spells seem to hit a "slump" at this point, failing to keep up with the power increase of other spells over this period. This becomes particularly disappointing if you are a big fan of blasting, as we know how few slots you get of the higher level spells, meaning you get fewer castings of poorer spells. That said, I have advised since day 1 that in 5e, blasting is a good hobby, but not a good primary focus for wizards.