Yeah, I agree that the terraforming tile model doesn't really make sense in reality. On the other hand, it does make for more interesting and balanced gameplay, which I value a little more.
Maybe there is a middle ground where we can have both realistic rules and interesting/balanced gameplay. Unfortunately, I'm at a loss here. For the choice to be meaningful, you have to be stuck with it for a while, but as you said, if you have the tech to terraform either of two tiles, why can't you terraform both?
I'm making a custom faction that has the capability to mass-terraform right away, but only to the point that they can turn bad planets into decent ones, or at least, that was the goal. Here's what I figured out while trying to balance this:
1) Terraforming eligibility is determined by what % of a hex is taken up by "land". Planet class has no direct effect, though planets with higher starting class usually start with more land, and these worlds also have a higher upper limit than naturally low-class planets.
2) Some planets have no land to improve, both high and low, so while this does mean you might get a class-22 that's as good as it's going to get, there are also planets so bad that there's simply no way to make them better than "Earth-Like", even with an Ultra-Terraformer. The custom faction star system with only one Dead planet as a starter is a good example.
3) Some planets start as high-class, but have a huge amount of empty land, so they can be improved greatly even with really crappy mass-terraforming technology. The starting planets of the Iconians and Iridium are extremely good examples. They have HUGE, empty, Pangaea-like continents, and if you have any kind of mass terraforming, you can jack them up to class 30, because they are loaded with hexes that have 100% land. It's even stronger than it sounds, because those tiles are also in nice neat groups that let you stack adjacency to the moon.
4) Mass Terraforming is EXTREMELY GAME-BREAKINGLY POWERFUL, even if you make the upper limits REALLY bad compared to single-tile terraforming. I initially had two levels of mass-terraforming in my custom faction, both much worse than the Ultra Terraformer, but available earlier. My custom faction would get the first level, then take off like a rocket and squash the entire galaxy beneath its heel, all before even getting to level 2. Even after many nerfs, that mass terraforming remained so unbelievably overpowered that I had to cut the second level out entirely and heavily nerf level 1, plus consider locking it behind Age of War.
You know the Ultra Terraformer that everyone gets at the end of the game? It has the same "power level" as the very first Soil Enhancement you get, except it can be built until there's no land left to improve. Even with my custom faction's best terraforming option only half as powerful as that, they could snowball out of control extremely fast, because every planet they colonized would end up class 22 or more.
I imagine the devs reached the same conclusion that I did: if you want there to be mass terraforming, you can't have multiple levels of it, because if you make one of the levels too weak, you can barely use it at all, so you can get utterly screwed by the RNG, and it's not fun to use, but if you make it even one percent too good, which is extremely easy to do because each percent increase makes it exponentially more powerful, anyone who doesn't have it is as good as roadkill to the guy who gets it first. It becomes an over-centralizing tactic.
So, when people say they'd have to re-design the entire economic system to give people mass terraforming, even weak mass terraforming, they aren't kidding.