This is a complicated question.
It’s not fair to just say “the dentist diagnosing more cavities is greedy and wants more money.”
I’m not saying that’s never the case.
Nor am I saying it’s always the case.
Let’s look at it closer, shall we?
Our teeth I have 3 major parts to them. The outer shell called the enamel, an inner layer called dentin, and a nerve in the center of the tooth.

2 different landmark depths of a cavity calls for vastly different treatments.
1. If cavity penetrates the outer layer (enamel) it is considered a cavity and the universally accepted treatment is removing it and placing a filling.
2. If bacterial cavities get beyond the first two layers (enamel and dentin) into the nerve; the universally acceptable treatment would be either a root canal or an extraction depending on patients desires.
Now what happens when those cavities are so close to the 2nd layer (dentin) like the x-rays above? Is it through the outer layer in all 4 scenarios? Hard to determine 100%, but it’s close.

That’s when the dentist has to take into account a number of things from patient’s age, dental history, other active caries/cavities, home care and oral health, accessibility to treatment, diet… and the list goes on. Diagnosing cavities really can be both simple and complex as you can see.
As a dentist, I would consider myself very conservative.
Borderline cases, I don’t want to treat unless it is necessary. I’d rather educated and let patients know their options. Whether the next in 6 months we have to do treatment or the patients health improves, it’s on the patient individually.
To me, it’s important to give every patient a chance to use my knowledge and experience with current science, research studies, and home care to have a healthy mouth.
Lastly, if you ever feel uncomfortable with a diagnosis, I always recommend getting a second opinion. Whether it’s my patient or another dentists’ patient, it’s your oral health. Asking questions is always a good thing.