In the "How Bad Can I Be" sequence, at one point, The Once-ler's body begins to grow bigger than the land around him; in other words, he, like his corporation, starts biggering.
As does his ego.
It seems a little odd that the Once-ler's business growing and him becoming rich was condensed into a three minute montage (cool as it was) in the film, since that took up most of the bulk of the story in the book. But the Once-ler is the one telling the story, so of course he'd want to skim over the part where he turned into a corporate douche.
The Once-ler's "it's a girl, isn't it?" comment. Ted's doing something crazy and possibly stupid to impress Audrey. The Once-ler got where he is trying to impress a girl too—his mother.
The reason for everyone's Plot-Induced Stupidity — inbreeding and the chemicals in the air and water (at least for the post-Lorax scenes).
Though it may be just because of how perfect Thneedville is supposed to be and how literally no one except for O'Hare, Ted, Audrey, Once-ler, and Grandma has ever been outside for generations.
The Onceler's business strategy of completely wiping out his source of income seems short-sighted, even from a profit perspective. However, the thneed is shown as a stupid short-lived fad that no one needs. Once the fad died out, his source of revenue would have ended whether the trees he needed to make thneeds still existed or not.
If there are no trees, where does the oxygen come from? The answer seems to be chemicals or possibly algae.
Algae is actually the source of the majority of oxygen in the atmosphere now(remember, most of Earth is covered in water and algae is everywhere in that water, the tiny bits of oxygen they produce adds up), not trees.
Phytoplankton does indeed provide a good deal more Oxygen than trees, but the movie establishes that O'Hare has lots of control and power because he sells clean air, which he makes. How does he make that air, and the very fact that he needs to implies something about the amount of Phytoplankton in the water. Perhaps they died out, and he keeps a culture for himself?
OK, so the Lorax sent the local fauna away to find a better place to live. Fine. But how many casualties were there before that happened, that we didn't get to see? I get that Dr. Seuss, generally speaking, is a Never Say "Die" kind of author, but at least a few of those Barbaloots had to have died at some point, whether by a tree falling on them, or starvation as the trees were chopped down. And, while we're on the subject, we see the humming fish climbing out of the polluted water. Unless they were actually amphibians (admittedly, not out of place, seeing as this is Dr. Seuss we're talking about here), they drowned in open air (since they were, you know, fish). Put another way, the Once-ler caused the extinction of the humming fish!
The Humming Fish were seen on land before the water got polluted, so my guess is that they are amphibious. Most of your point still stands, though.
Biggering (the cut song) did imply that some of the animals were dying.
I always took this differently. I always inferred that thanks to Dr. Seuss's Never Say "Die" nature, "being sent away" was a euphemism for impending extinction.
The Once-Ler spent HIS ENTIRE LIFE regretting his actions, you can only wonder the kind of thoughts he had over the years....
Mr O'Hare was last seen flying uncontrollably out of town into a barren wasteland with a rocket strapped to his head. There are a number of things here that could go bad.
Mr O'Hare sold air. The very thing people need to survive. So what if you couldn't afford air? Mr O'Hare has the power to deny Thneedville survival.
A cut verse from the opening "Thneedville" song has O'Hare gloating "everyone around here works for me". Some fans have suggested that this verse was cut precisely to remove fridge horror: namely, the implication that mass unemployment would ensue after O'Hare's banishment.
In the movie, when the Once-Ler is informed that its too time-consuming to harvest the tuffs of the tress when they're standing, the mother's first solution to have the trees cut down. As opposed to, oh I dunno, using a ladder.
This makes sense since it's the mother saying that they need to cut the trees down she hates the idea of waiting for her money and would never waste time working.
Why didn't O'Hare Air just wait for Ted to plant the seed and then douse it with a good dose of herbicide? Or, if he wanted to keep the tree as a source of oxygen, you'd think the company could just dig it up in the middle of the night and take it to some secret hiding spot.
Or better yet, just BUY it off of Ted. He's the richest man in town; Ted could name his price.
By that stage, Ted had heard the whole story from the Once-ler and reached a stage where he genuinely cared about the trees. He probably wouldn't have accepted an offer like that anyway.
Why would herbicide even exist in a town that hasn't even had a plant grow in 20 years?