This semester I took a class called Competition, Regulation, and Business Strategy. As you can probably guess, the professor talked about monopoly all the time. It was a constant stream of explaining how firms seek to become monopolies, encourage anti-competitive behavior, and how they exploit consumers to maximize profit once they've achieved it.
The last bit is the standard answer to what makes a monopoly bad. Economists tend to look at competition as the be all end all, the saving grace of the market economy that makes the world go round and monopoly stands in opposition to it. After all, in a true monopoly, one firm dictates the quantity supplied and the price. Dictating prices intuitively seems like exploiting consumers, as firms maximize profits and they'll charge whatever they want.
The problem here is a misconception about the way market share works. Regulators worried about firms 'taking' market share from each other are plagued by the scarcity mindset. They constantly view any addition as inherently also subtractive. This narrative is common when talking about wealth, they have this perverse notion that there's only a set amount of material wealth that just gets shifted around based on how much businessmen can scam each other out of it.
The absolute absurdity of this claim should be fairly obvious, but more than just being false, it's harmful. If it were true, then technology would not exist. I'm not saying high tech stuff wouldn't exist, and life would be similar but we wouldn't have iPhones or TV's, the actual category of technology would have to not exist in order for this to be true. Technology is anything that improves upon or creates a new process. It's only adopted if it's useful, and if it's useful then it's doing things better than before, meaning it uses less resources or creates more for the same inputs. Either way, more resource (physical or intangible) makes people richer.
Regulators see market share in this same way that most people see wealth, when in fact they should both be seen through an abundance mindset. Achieving monopoly doesn't mean you can stop worrying about competitors. In fact, in a real free market, it would be a king of the mountain situation. Sure the person on top has the upper hand, but they're not insulated from new competitors, because you can always create new market share. If an industry is monopolized and the company is jacking up prices, new competitors can just undercut them and establish their own share of the market.
What if the dominant firm adjusts their conduct to take the new firm's market share? Good! This means they're creating more value for their customers. Peter Thiel talks about this in his book, Zero to One. The goal of any entrepreneur should be to create a monopoly. Make your product so good that nobody wants to get it from anywhere else. In a real free market, this is the only way to create a monopoly, so tell me, what's so bad about that?